I have a PowerMore OHV 173cc power mower, it starts just fine but stops within two or three seconds, this just started. I have not had any problems prior to this. I have changed the spark plug but problem still continues.

This sounds like a blocked air filter-try to start it witout the air filter-if it runs, you need a new filter.

if it is still no good remove the fuel cap-there should be a small hole in the fuel cap-this should be open, try to start it without the fuel cap-if it stilll refuses, you may need to check compression, but try those things above first^^^.