How can I fix my B&Q Electric Chainsaw?

I have a B&Q Performance electric chainsaw which electrically is fine,plug it in press the trigger and the chain races round, but as soon as I put any pressure on the to chain, i.e. attempt to cut a small log the chain stops revolving and will not go again until the pressure is eased. The chain saw is about 4 years old and has not had a lot of use. Any suggestions please .