If it's a Multi Point Lock (common on most UPVC doors) you'll need a locksmith to open door and fix problem.
Our website gives you an idea on what you can expect to pay, approx £45 to overcome failed lock and fixed quote once fault identified. Steer clear of national callcentres as they'll try and charge silly money.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix windows and doors.