it could be one of two things: seized engine, or chain bunched up under the drive sprocket cover. a siezed engine can be "fixed" but it'll never run right again so you should just replace it, if it's the chain then you should adjust the chain tension
I've also seen debris in the clutch side gears cause an engine to lock up, in which case you should clean the debris out and replace the gears if the damage is bad.