Cam Timing - Engine Won't Turn!

[Fazerider]

Have you got the inlet and exhaust cams the right way round?
Having the clearances slightly wrong won't cause the engine to jam.
It must be grossly mistimed if a valve is meeting a piston and, if that's what's happened, heaving on the crankshaft hard enough for the chain to jump could well have bent the valve unless the bottom sprocket was knackered.