Toro self propelled mower will not start.spark plug and air filter replaced. What else to cause this problem?

Was it left all winter with old gas in it turning into varnish?

Old gas, bad coil, have you checked for spark and gas flow to the carb?

If this is the first time used this Spring, and as already said earlier- is the gas fresh, whether left in mower or storage container over Winter? Air filter?
Depending on the age of the mower, many other possibilities.
This may be helpful (I didn't watch all of it and cannot guarantee its accuracy): Carburetor Cleaning & TUNE-UP of Toro 6.5HP Lawnmower with Tecumseh Engine - YouTube (http://www.youtube.com/watch?v=8h2O4QsUZZo)