07 yz450f. today I was riding and all of a sudden the bike made screeching noise and locked up, once i was off the track I tried to turn it over but the kickstart was frozen. I was convinced It was blown up. when I got home I washed it and tore down the top end. I was stumped, I found nothing wrong. the timing was good, chain was tight, no bent valves, piston, rings and cylinder looked great. Then I started turning the bike over to see If I could find anything wrong with the bottom end. nothing! so I put the bike in gear and spun the wheel by hand to see if any of the gears were messed up and once again nothing. what the hell happened? what should I do?