Today my knee gave out on me and i collapsed. Now it hurts when i walk on it and i have a really bad limp. it feels like I have somthing pressing on it.
What could be wrong with it?
What should I do?
I have gym tomorrow so what should I do?

welcome to the bad knee club. It may mean that the soft tissue is inflamed between the knee joint.
If you have to play in gym perhaps you should tape it up once you get dressed. It may get better on it's own or it might get worse.
My right knee gave out on me a bunch of times a long time ago, it hasn't since, but sometimes it feels like it will. Wear and tear & sports injuries & extensive work around the house at ground level stresses out my right knee. My left knee ain't too bad.
there are exercises to strenthen knees without going to a physical therapist. You should be able to find them on the internet.

Go to the dr or wear a knee brace in the mean time, I've been their before but much much worse than you, I even had to walk up the stairs it was such agony!!!
Anyways if you can just miss school tomorrow, If u don't want the dr to prescribe something weak like ibuprofen(Motrin) just say you tried it and didn't work, seriously he gave me ibuprofen and didn't even work?!!!
B/c their hesitant to prescribe narcotics to teen well some..... Anyways just try not to walk