Are the end of the valve stems equal when both valves are closed? Or is one noticeably looser than the other?The most likely reason is that you have a valve seat out of position and the valve cannot seal. Your other choice is that the piston no longer travels. Hope for option 1. Or that your thumb has a hole in it...

another thought...

the intake valve stem may be gummed up with residue from ethanol fuel. the valve may be operating part of the way, but not closing.

if this is the case, some techs spray carb clean on the valve stem to loosen it up. This repair may not last. the best repair is to remove the cylinder head and valve for cleaning.

***" I'm thinking maybe a bad head gasket?"***
Absolutely not!
While a head gasket is not a good thing at all, it is also not something that causes sudden stopping of the engine.
And while a failed head gasket causes a REDUCTION of the compression, it DOES NOT cause a total loss of all compression.
Head gasket failure in these engines results in copious volumes of blue/white smoke out the exhaust in worst case scenarios.
The 2 possibilities offered by tomplum (3 if you count the thumb) are most likely.
And while intake valve sticking is sometimes a problem in these engines, I have never heard of a valve suddenly sticking open after the engine is already running and warmed up.
Intake valve sticking open is something that occurs because the intake valve just happened to be in the open position when the engine finally stopped rotating when cut off at the end of the previous use cycle/event. The engine cools with the valve open and makes it possible for varnish on the valve stem/valve guide to "set hard" and immobilize the valve (at least temporarily).
When the intake valve is open like that, the next time the engine is cranked, the valve may not "unstick" as soon as the cam lobe runs away from the tappet. This often cause the push rod to fall out of position and come loose from the rocker arm, or worse, it gets trapped on the edge of the rocker arm cup or ball and if that happens, the result can be a bent push rod, a bent valve, a bent rocker arm, or all three.
A sticking intake valve seldom occurs without leaving some damaged parts in its wake.
You need to remove the spark plug and look into the spark plug hole with a good light and watch the piston while you rotate the engine by hand. If you can see the piston moving up and down when the engine rotates, the connecting rod and crankshaft are in good order.
If the piston does not move at all, the engine has suffered a rare form of connecting rod separation. You did not state whether there were any "unusual noises" accompanying the stopping of the engine, but connecting rod separation is usually a very violent and noisy (sometimes with shocking visual effects too) catastrophic failure. But, there have been a few cases where the rod separates in a relatively quiet manner.
The possibility of an exhaust valve seat ejecting from the head and holding the exhaust valve open is the most likely thing to have occurred, considering your description of how/when the engine stopped.
If this turns out to be the case, you are also likely to learn that it was caused by either a build up of grass clippings inside the cooling air duct/shroud around the cylinder, or that mice have built nests inside the cooling air duct.
The overheating of the cylinder and head is what causes a valve seat to eject from the head.

Pulled engine, pulled spark plug and looked in cylinder. Of course my luck...piston is not moving! I drained the oil, removed the dipstick tube, looked inside and there are large hunks of aluminum so I'm assuming failed connecting rod. I'll disassemble tomorrow to see if it's worth rebuilding or if it will be cheaper to buy a new one for $550.00 from Northern Tool.
Thanks for all the advice and help!