Not sure what It could be. Can yall give me an Idea of what it could be ?

What is your question?

I have a 96 Chevy Tahoe 4x4, I've been looking at it for the past week and can't figure out the problem. I was driving to school and i was stopped at a light and when it turned green I hit the gas, and heard a big thud and then I couldn't go anywhere. The engine starts just fine but I pushed it outta the way and tried putting it in reverse & you could hear it tryingto catch but still didn't move. I checked the the tranny for broken gears & metal and what not but there was nothing in there. I'm not sure what else to check or what it could be.