I had exactly same problem with this type of saw, used it once but couldn't get it to start again a few months later, spent ages trying to get it to start with pullcord.
Emptied the fuel tank and filled with pump-fresh super-unleaded (with 1/40th 2-stroke oil) and it started first time. Have been told petrol "goes off" within a few months. Hope this works on your saw.
Mark
November 2011