I have a 1992 Polaris EFI 500 sks, left side cylinder is not working... HELP!?

I bought this sled about a month ago, it has been rebuilt and sitting for 2 years, worked fine for the first few weeks, I was driving home and it shut down on me, now only the left side cylinder is working. I bought a computer (EFI) becuase thats what the people at polaris said to do, we changed it and it still doesnt run. Anyone know of anything else that might work?? please help