Pull the plug and see if the ground electrode is smashed over or there are little silver dots on the plug. If so, that's the signs of piston failure. If the plug looks good, put the plug into the boot, lay it on the head, kick it over to check for spark. If it has spark, do a compression check. Do NOT take the head off to "have a look", without 1st doing a leakdown test. If you see the plug as mentioned above, pull the exhaust and look to see if you have a broken piston skirt. If the exhaust side of the skirt is broke off, it will not start.