PTO switch still on?

If not' look into Electric PTO clutch going bad stuck in the on position?
If its manual PTO lever did you move it to the off position? We've all been there done that before!!..lol But maybe your not old enough to do that yet?..lol

My guess is your PTO or its switch is bad. Also it is still in the on position even if its turned off? Killed engine when it failed.

Could also be as mentioned rope/wire around main shaft.. Seen it before myself.
Good luck hope its a cheap easy fix. Meaning hope its not the PTO clutch...grrrrr!$$$ Easy fix, a bit costly. YouTube vids will show you the way here.

Quite possible you have a wrapped wire or string around the crankshaft, between the blades and the deck. I have that issue with Craftsman 3-blade and it' s generally either fence wire or bailing twine blown over from the adjoining field. Undo ALL the sparkplug wires before shoving your hand in. Might even have to unbold the blades to get a good access if that's the issue