I just rebuilt my top end on my 1984 honda xr250 dirtbike and i just put my new cam it the head and went to?

start it and it kicked over 5 to 6 times easy like it had no compression and then the piston got really tight now wont turn over i had it bored out 77mm over put a brand new wiseco 11:1 compression piston in it the new cam what could it be i even took the valve cover back off to see if the piston woul release but it didnt y please somebody help me i dont really wont to take the head off cause i just put a new gasket on and dont wont to screw it up what can i do to see what my problem is