How old is the mower ? Could be lots of things. Bad or old gas, fuel lines gummed up, dirty air filter, bad spark plug , and a few others. If it's old, it could have slipped a ring but they usually won't start at all then. Go through these things and make sure everything is clean and right. If it still does that, then take it to a mower shop and let them check it out. I'll bet it's one of the things I just mentioned.