What did I do to my knee?

A few days ago I was playing basketball in gym and was trying to knock the ball back into play, but my knee stayed in place and made this nasty liquidy popping noise and I immediately feel down and could feel the pain. My friend tried to help me up, but it made another popping noise and I feel back down again. I have an MRI next week to find out what exactly it is, but if you have an idea please tell me. Also, these past two days, I have been able to hobble and limp around on my bad knee without a crutch or with only one. It almost feels permanently bent because when I try to straighten it, it feels like it's going to pop again. There's only a few positions where it feels comfortable, so if you have any ideas, please help! Thanks =D