+1 on the clutch adjustment.
Also, if you have been working on it, perhaps a bolt or nut could have fallen and landed on the chain and gotten wedged in the link. This would cause enough friction to kep the sheel from spinning. (yes I had this problem once)
Can you tun the back wheel a little, as in the amount of slack in the chain?
Should be a little give in it, but not much.
If not, back brake might be locked/rusted/out of adjustment.