whenever this kinda thing happens, my first idea is to connect what just happened to the new problem.. just seems logical.
Why did the chain pop off? did the piston seize?
Did the chain damage some piece of ignition related wire while it was flopping around?
Is the chain now binding or is some other part of the drive too tight?
Did the engine over-rev when the chain popped off and do internal damage to something?
Were you working on the bike or fixing something that could be related to this start problem?
If you actually have fuel and compression and spark, the engine will pop.. Use starter fluid to verify ignition and to rule out carburation trouble.