my brush just doesn't stay switched on for more than a couple of seconds?

Oral B sonic complete oxyjet centre OC S18 was a gift that got no use until this xmas when i moved so although its an old model it has only been used for a week before it started to only stay on momentarily???
the shop it was bought at wants receipt before they'll send it back to Braun & as i can't find it i am stuck its an expensive article to only work for a week...