pull off a valve cover and look for anything outta place.. bent pushrods, broken rockers, springs.
if they all look fine i would turn the motor over by hand and see if the rockers move at all. if their not moving then you have an more internal issue with the timing chain. which means you gotta tear the front of the motor back apart and find out what went wrong.
my car had the same symptoms, turned out the distributer was cracked, and it cocked it sideways where the rotor wasnt close enought to spark the plugs.