Small Engines (Lawn Mowers, etc.): lawnmower engine, starter fluid, model numbers

Question
I have two mowers that I had serviced and the guy brought them back, cranked em up and the next time I went to use them niether one would crank.  Yes they have gas, lol, but they just won't start up now.  Any ideas?

Answer
 Hello Robin:

 Send me the Model Numbers Off of the Mowers and and Off of the Engines. I can Better Assist you with these Numbers as I can Look Up the Diagram. There are Several Possible Causes for your Problem. No Fire at the Plug and if they are Riding Mowers, you have the Safety Switches to Consider also. Try Spraying a Small Amount of Starter Fluid into the Carburator Throat and try to start the Engine.  What Does the Engines do. Let me know what Happens, Please. Thanks.

 Respectfully

 John