Here's a list of things to check: valve seizure,piston seizure,crank seizure,large and small end connecting rod seizure,camshaft seizure, Transmission gear or crankcase bearing seizure,kick starter return spring broken,kick ratchet gear not engaging. I'd start with the last 2 and go from there. GOOD LUCK!!!
Does it turn over, if not check previous items. If it does check to see if you have spark.If you don't check your ignition coil. .19-.23 ohms on the primary side and 2.5-3.7 Kohms on the secondary. You may have water in the magneto. That happened to ours.Also check to make sure the air screw and idle are correctly adjusted.Double check all elec connectors . There may be dirt/mud in them. I hope this helps.