bicycle pedal stuck when cycling

[soelydim]

I was cycling nicely to school and when I was going to cycle home after class the bicycle completely gave up.

The pedal, will not move at all when I sit on it. It however would turn if I kick it down while standing but whenever there is increase weight on it, the pedal just wont budge. Neither does the chain.

I tried putting the bicycle upsidedown and spin the pedal. The chain and the pedal works perfectly. But when I sit it back up and sit on it, the pedal wont move again! However when I sit on it and let it go down the hill, the chain and the pedal would spin as usual; even after I reach to a flat surface I could still pedal it. But once I brake and stop, the pedal wont spin again.

I don't know if this is how the bicycle is trying to tell me that I've put on too much weight.

Does anyone knows why it's doing this?

 

[dabac]

Is it a coaster (=back-pedal) brake rear wheel? In that case I suspect that either the clamp for the reaction arm has come loose, and/or that the bearing on one side has come loose and is alternately pinching/seizing. Probably fixable, but not easily talked through over the internet.