I have a briggs and startton lawn mower and the

Pull the plug. MAKE SURE THE PLUG IS COMPLETLY REMOVED AND SEPARATED FROM THE MOWER!! Remove the cover with the pull start attached. see if you can turn it by the flywheel or the blade. If you can, you can just have the rewind repaired.
If not, then you have a probable bent or twisted crankshaft. Most likely not worth repair.

Could be a number of problems, including seized engine. Check to see if the engine break is releasing when you pull on the break release lever. Another check is the possibility that the rope has become tangled in the housing in which it resides.

unleaded fuel

did you hit something while mowing the key way between the flywheel and crank shaft might have sheared it will act like the blade is missing pull back on you while pulling the rope

go to replace pull cord follow instruction

Take the plug out and try to move the blade. If the motor won't rotate, the problem might be with the motor and the cord can't move because of it.
Gary

Pull the spark plug out and see if it isnt hydro locked. Someone might have overfilled the oil or the carb float may have stuck open and filled the intake with gas!

If the recoil is still stuck you can remove it from the mower and try it without any load on it. Still stuck? The unit might be rusted solid!
You can spray a little penetrating oil into it but be careful.....the oil will soak the pull cord which should be cleaned and degreased immediately following using the penetrating oil.

Take off your cover to the pull cord, the cord probably just got wrapped around the pulley. If you run into trouble with taking off this cover go to smalleng.com they will walk you through the process off pull cord removal and installation. Good Luck PAL.

make sure nothing is jammed around the blades...if thats ok, remove the pull cord housing and try and spin the engine. if it doesnt turn, the motor is locked up and needs repaired