Could be several things. Could have locked up from overheating (discolored paint on cylinder head or exhaust cover). Could have been a failure in the gearcase (how's the oil look? Any metal chunks?). Could have been lack of oil in the gas (running 24:1 mix?). Could be liquid locked (saw this once with a pressure tank motor. Tank was pressurized and it forced fuel into the motor past a stuck open float needle in carb, filling the crankcase and one cylinder with fuel, effectively locking up the motor). Could be water locked - blown head gasket allowing water into one cylinder? Time to check some things out...
