Remove the plug and see if you can rotate the engine. If so, it was just hydraulic lock - crank it until the oil/fuel is expelled from the cylinder, then use a new plug.
If not, remove the cowling (4 bolts) and see if the flywheel can be rotated. Use some force - you have nothing to lose. Go BACKWARDS first. About 1 time in 3 you can break a siezed engine free. Check the oil - is it clean? If so, the oil has been added AFTER the engine locked up. If you DO manage to free it up, leave the plug out and crank it on the starter for AT LEAST 5 mins (in 1 minute bursts separated by 5 minutes 'cool-off'). Put the plug back in, and try it. It will probably run, but may well be completely stuffed - if you can still see the mower through the smoke after 5 mins running, you may be lucky.
Be warned - it probably won't last long - the bearings will be seriously scored, and may even have spun in the housings.
Good luck!
phil_saunders(a)bigfoot.com
Phil Saunders, January 2011
