Cause
Restriction in kick starter, top-end or transmission.
Caution, do not use excessive force in an attempt to free the lever.

Free transmission
Rock vehicle forward and backward while trying to shift

Diagnose
Turn key to off position

Remove spark plug to eliminate cylinder compression.
Remove flywheel dust cover
Try to turn flywheel
If flywheel is hard to spin the restriction may be in in the top-end .
The kickstarter gear spins the crankshaft
If a part is lodged in this gear the crankcase needs to be inspected.