If it had hit something , with the blade , it is common for the key between the flywheel and top shaft , to sheer & the flywheel move ( rotationaly ) in respect to the top shaft . Rhis throws the ignition timing off .
Pull the rope starter assembly off and check . I made a puller to get the flywheel off ( after I removed the nut at the top shaft ) .
The keys are inexpensive and available at Ace Hardware & other places that sell mowers & supplies .
God bless
Wyr