OK well there are two things to check.
Take off the blower housing and check that the flywheel has not spun on the crankshaft.
If it has not the two halves of the key way will align to make a perfect square.
Next pull off the rocker cover , this will generally destroy the gasket so buy a new one before you start.
rotate the engine slowly using the flywheel with the spark plug in and removed.
Note the movement of the rockers, at the top end of the stroke ( piston coming towards you ) after the inlet valve has closed ( lower one ) both of the rockers sholug be flat across the head and both of the rockers should be the same distance off the the head.
If the inlet is a bit lower then it is either stuck or the valve seat has come loose.
You can confirm this by removing the carburettor and shining a bright torch down the inlet hole.
If you can see light when looking in the spark plug hole then the valve is not closed properly.