Sthil S80 - won't start - why ?

My Sthil S80 won't start . When I pull the cord it turns over but won't fire . I've replaced the spark plug and have checked the settings on the High and Low speed screws and they seem fine . I've had the S80 for about 6-7 years and it's been brillant - it doesn't get a lot of use - maybe 12 hours a year - and this is the first time there's been a problem .