Why won't my lawn mower start?

It was running fine for probably 20 minutes while I was mowing. Then I had to cut it off to move something out of the way. When I tried to start it again, it sounded good for a second and then sputtered out. There's still gas, so I'm not sure why it would suddenly refuse to start if it was working fine before. I tried priming it a few times and that didn't help. Any ideas?