The mower has oil in the bottom, like a car.  When you turn it over, the oil goes to the top and into some bad places.

It flows into the air filter through the breather and plugs up the filter, and into the cylinder around the piston, and finally into the muffler.

Take off the air filter and take out the sparkplug. Then spin the engine to blow oil out of the cylinder.
Clean or replace the sparkplug, and leave the air filter off for a test try.

Prime the engine and give it a pull. If it starts, it will probably smoke up the whole neighborhood and you can either let it burn out the oil or remove the muffler and wash it out.

Finally, if you got it running again get it a new air filter before doing any cutting.

Sometimes flipping it over stirs up junk in the gas tank and float bowl. If it runs for a couple and second and then shuts down it means the carb probably
needs a clean out, too.

To work on the bottom of a mower, the best way is to tip it back on the handle if you can. Don't forget to remove the sparkplug before sticking your HANDS down there!