If you cannot open the door by turning the key in the cylinder, drilling the cylinder will do you nothing. You have to either break out the insert (if it is an A2 under-handle insert), or cut out the fixing screw with a sharp screwdriver and knock it out inside. Later, use a curved screwdriver to open the door lock, same after breaking the cylinder. The breakout consists in removing the signs to have a bare insert (remove them to cut the screw) and either hit the inserts to the side with a hammer, or squeeze the inserts with a Frenchman and break them to the sides (I have a special device made), also in this case it is needed a hook made of some soft screwdriver or rod to open the door lock. Breaking the cylinder is the least invasive, because it will destroy the lock and perhaps the door, it is the last resort.
