It happened while I was riding, it tensed up really fast while I was pushing the pedal down, and then all of sudden it loosened up and the chain flies off the chain wheel.
I put it back on, and then when I tried to ride it, the back wheel wouldn't rotate. It's like spinning the pedal backwards, but only forward.
Any idea how to fix it? The freewheel is rotating, just the actual wheel itself isn't.
Cf. the other threads on this subject (tire won't engage and whatever they called them): The pawls in the freewheel / freehub might be gummed up. You could try to flush them out with some oil.
If it is a freewheel I'd probably replace it (how badly is it worn? and the chain?) as they are quite cheap.
I read the thread, and removed my back tire.
The freewheel doesn't rotate with the tire in either direction. But I am 99% sure something is broken, because when I try to rotate it, I hear a grinding sound, and the sound hasn't always been there, been like that for two weeks, it started happening recently.
I tried taking off the freewheel off the tire, but I don't think I can, probably have to take it in to a bike shop, or just get a replacement tire, unless it's fixable...
What do you guys think?
Well, you cannot remove a freewheel without the appropriate tool (well, there's some tricks, but they don't work that well). Find a bike shop to remove the freewheel. I'd pick up a new one there, too. Shouldn't be too expensive.
Well, that is assuming it is a freewheel not a freehub. If you have a broken freehub, you can maybe replace the freehub body (depends on the brand). Otherwise you have to get a new wheel.