have had a couple of these with being a gardener and had one which went the same way as yours sounds. when you depress both red handles give it a little shake - can you hear something rattling around inside? if it goes on then off or you can hear rattling inside then it will be a very small white trigger inside which has come loose. it's not easy to fix and if it's still under guarantee take it back to the shop. if out of guarantee you might as well try to fix it yourself - lot of screws to loosen and the two parts need to be prised apart which ain't easy - once opened, carefully, should be self- explanatory how to get it to start. hope this helps!
steve smith, August 2009