Ryobi petrol strimmer won't start?

Hi all, my 25cc Ryobi Petrol Strimmer (multi tool version) was running a week or so ago, died when it still had fuel and will no longer start. I have replaced the spark plug, cleaned it a little and even partly dismantled it before assembling it again but nothing changes. It will turn over, almost start to run (sometimes it does but only for seconds) and then dies. It has given me 4 years of great service but I'm stumped as to why it won't start - any ideas?