I left my lawnmower outside for awhile and now it won't start. Can I fix it?

We've had unseasonably cool weather and I left my push lawnmower outside for about 3 weeks. I tried to start it up today and it started right up like it normally does, however, I got about 50 feet and then it just died. I could not get it going again. I let it sit for awhile and now I get this weird sputtering that lasts for anywhere from 5 seconds to 20 seconds before the engine dies again. I didn't know what to do so I changed the fuel out thinking it was water but that hasn't changed anything. Like I said, now it's just sputtering always on the brink of dying and after 15 seconds of this pathetic sound then it dies. Any ideas of what it is and how to fix it? Thank you.