This is likely to get lengthy but I promise the information is worth reading. Though, if your budgie is actually bleeding, breathing heavily, has a strange lump, vomiting, shivering, staying puffy, not moving, hanging around the bottom of the cage (when doing so is out of his/her character), not eating, losing weight, being unusually quiet, not using a limb or not using one normally, or pretty much exhibiting any behavior that you find odd or different, at the very least call your avian veterinarian as soon as possible. Post here AFTER you've called a vet (and hopefully made an appointment that is soon). Time is almost always of the essence with these little birds. It may be no big deal but it could very well be an emergency. No one can advise you better than an avian veterinarian who has gone through nearly (or more than) a decade of schooling for what he/she does. We are not veterinarians or even experts on the health of budgies or any other bird. We can take a stab at the cause of your budgie's problem based only on our experiences and the experiences of those we know and/or have read about but in the end, we're not there to treat your bird. Our opinions are merely educated guesses and not nearly as educated as those of a true avian veterinarian.
If you don't know who the closest avian veterinarian is, try using some of these links to help. If they don't point you in the right direction, give your local small animal vet a call and ask them to refer you. Don't let them talk you into taking your bird into them as many of them see birds but aren't actual avian vets. You and your bird will be much better off going to someone who specializes in birds though in a dire emergency any vet is better than no vet at all.
Budgies in the wild that show signs of illness, injury, or other weakness are the first to be picked off by predators. Because of this, they instinctively "hide" their troubles for as long as possible. If you've ever watched a budgie die (and I hope you've been spared the sight) you'll know that until the point where they can no longer move, they attempt to look as normal and alive as possible. It's extremely sad to watch, but it's what they're designed to do. As a result, a sick budgie will make itself appear healthy and by the time you actually notice that something is up, the bird is feeling extremely bad. Sometimes they show no signs until they're on the brink of death. This is why ANY change warrants a phone call to your avian vet at the very least. It costs nothing to make the call (well, depending on your phone plan) and your vet can let you know if he/she thinks the bird needs to be brought in and when. Making an appointment as soon as you notice something amiss is the best course of action but a phone call is better than wasting time trying to guess yourself. Which brings me to another good point: Googling the symptoms or asking people on the internet will usually end in the answer "It could be any number of things." This is because symptoms of illness in budgies are very general and can be pinned on many many different things. It's pointless to try and guess the illness based on symptoms and even if you could, you can't treat it without a vet.
What can I do at home?
Until you get your budgie to the vet, separate him from all your other birds. It's a good idea to keep a small spare cage around to use as a hospital cage. Put this in a warm, quiet room. If your budgie is looking very puffed, he/she is cold and trying to stay warm. You can put the cage in your bathroom letting the hot water run in the bath/shower so that he can get warmth from the steam. You can also place a heating pad under the cage. Set it on low and underneath a towel. Place the cage on top of the towel. Heat lamps can work as well but with any method, make sure the budgie isn't going to get uncomfortably hot. We're not trying to cook the budgie, just keep the chill away.
The perches in the hospital cage should be low. If your budgie is having trouble perching, place something soft (that won't snag his toenails) in the bottom of the cage. Food and water should also be offered at the bottom of the cage so that they're more easily accessed by a poorly budgie.
Won't the trip to the vet stress my budgie too much?
The trip will stress your bird but it won't kill him. On the other hand, the illness/injury afflicting him could very well kill him. Which would you rather be: Stressed for part of a day or dead? The only budgies I've heard of dying on the way to the vet are the ones that were dying before they even got started on the trip.
What if my budgie starts looking better?
You still need to make the trip to the vet. Many budgies start perking up only to die later anyway. At the very least, call your avian vet and explain the situation to him/her in detail so that they can make this judgement for you. Don't simply assume it was a "24 hour virus." Budgies are not people. They're not even mammals and this should be kept in mind when we think of their health and behavior relating to their health. As much as we'd like to relate to them in this way, we have no idea what they're feeling, especially with an animal that instinctively hides its bad feelngs.
What if I can't afford the vet and/or my parents won't take me??
At the very least, call your avian veterinarian for advice. But do keep in mind that to deny an animal health care is cruel and unfair. Before you bring a budgie home make sure that this is something you can provide for him/her. It is part of basic care and should not be considered an option. Too many budgies out there suffer because no one wants to pay the vet bill for a bird that costs so little to begin with. Your budgie depends on you for all its needs and medical care is one of the biggest.
Many vets are willing to work out payment plans for people who are unable to pay the entire vet bill at the time of the visit. Don't just assume that you won't be able to afford the visit. Be straightforward and honest with your vet about your budget from the beginning. Also, make sure that your vet knows how badly you want your bird treated and how much it means to you. A caring budgie owner is something they don't come in contact with nearly as often as they should. Sad, but true.
Lastly, if you don't have a job, can't get a loan from your parents, and your vet won't work out a payment plan, look around your room. Most of us has something worth a little money to sell. To love an animal is to do and give anything you can to keep it healthy and happy. To merely like an animal is to give it food, water, and a place to live that may or may not be up to par. I for one can't understand why a person would keep an animal for any reason other than loving it but I've seen many who do just that. If that's the case with you, it would be a good idea to reconsider owning a pet that your heart isn't completely into taking care of.
We want your budgie to get better and live a full life but in the end, that's up to you. If your budgie is sick or hurt, get it to a vet! At least CALL the vet before posting here. I know in a moment of panic you do all you can to figure out what is going on. But I promise you that it's faster to dial a phone number than it is to type up a frantic post on a bird forum. Especially if you put your avian vet on speed dial or tape the number to your phone. And you should...right now! Finding an Avian Vet