It isn't usual for an engine to stop running due to loss of compression, unless it happened suddenly due to a catastrophic failure. I did achieve that kind of catastrophic failure once, with a motorcycle. Like nearly all old motorcycles it had no air filter at all, and I'd spent my last few cents on fuel. Because it had a black fuel tank and it was very hot I took my white shirt off and draped it over the tank, to reduce evaporation. After about 15 miles it stopped. A quick investigation revealed it had sucked my shirt in through the carburetor, through the intake valve into the cylinder, and burned nearly a foot of it away. Some of it caked under a valve and deprived it of all compression.
If your engine lost compression suddenly, taking in foreign material through an ill-fitting air cleaner could well be the cause. It should be obvious, though, when you look at the fit of various parts of the air cleaner, and you should be able to demonstrate the lack of compression by doing the Briggs & Stratton compression test: flip the cutting disk backwards against compression (with the spark plug disconnected, of course).
First find out whether you have compression, and second find out how you lost it. If it happened suddenly while mowing yard-high grass without a working air cleaner, you can have a fairly strong suspicion you have imitated my motorcycle-and-shirt trick. If so, it is no big deal and you should be able to fix it yourself.
You will need to remove the cooling air cowl, and the cylinder head. Then you need to remove the valve chest cover, and remove the valve retaining cotters and valve springs. Having done that, you inspect the seating area on both valves and seats to see if they are in contact, or are pitted, burned, or built up with crud (the latter is likely in this case, if you had an air cleaner failure). Apply some valve grinding paste to the sealing area on a valve, attach a suction cup device to the valve head, press it against its seat, and twirl. After you get tired of twirling you draw out the valve and look at its seat. It will have been lapped away a tiny amount of both valve and seat, and there should be a contact surface. You continue the process until you have a decent contact area all the way around both valve and seat. Then you put some ointment on the blisters on the palms of your hands and start on the second valve.
If you have sucked crud into the engine in sudden, catastrophic quantities, it will be easy to fix compared with a burned valve. Also, if it happened quickly it may not have damaged the rings and bore very much. Once you get the valves sealing, take a look at the bore and see if it has scratches running parallel to the piston movement. If they are severe, it will have to be honed or rebored. If not, you can clean the combustion chamber and piston-top, reassemble the valves, put the head back on, and see if it has decent compression. If it has, fix that air cleaner problem before you take it back out into the paddock, and promise yourself to be more attentive in future.
