Diane,
Most likely you have a fault with the valve that cuts the motor when the trigger is closed.
This re establishes the power on a falling pressure when you squeeze the trigger.
You will have to strip the machine to establish a valve or switch fault, then go on a spares site like espares.co.uk for an exploded view and part identity.
t.r.services
July 2009
I can answer this question...
How do I get the power to start again after I release the trigger?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix general d.i.y.