CR125 Will not Turn over at all

We have a 1988 CR125 that will not turn over. The kickstarter will
not budge.

This is what happened, my son was riding the bike around the yard one
evening last week. The bike ran fine and has run fine for months. He
turned it off and parked it in the garage. Then the next evening he
went to start it and the kickstarter would not budge.

I figured the piston may have seized somehow, or the ring was stuck
so I took the jug off. It slide off with no problem. Piston looks
good, jug looks good. We just had it bored, honed and put in a new
Wiseco kit last fall. There doesn't seem to be excessive play in the
rod bearings.

With the jug off, the crank still would not budge. Putting the bike
in gear had the same effect, the crank would not move therefore the
wheel would not turn. I put it back in neutral and after some playing
with the kicker and rod, I got it to turn. It turned a couple of
times, then stopped again.

I then took the right side cover off to see if I could see anything
obvious in the clutch, kickstart gears or primary drive gear. Nothing
obvious there. I took off the left side cover to see if there was
anything obvious in the rotor, again, nothing I could see.

So now I have 2 questions. First, what could possibly have happened
overnight like that to go from running fine to not being able to turn
at all? And secondly, if I take the clutch and clutch center off, will
I be able to narrow down the problem to one of the kickstart gears,
clutch itself, crankshaft or transmission? I know that if I can narrow
it down to the crankshaft or transmission then I would have to go into
splitting the case to see, but I want to eliminate all other
possibilities before we go down that path.

Any insights would be appreciated.