Can not open mortice lock with backdoor key?

Does the key spin continuously in the cylinder? Or does it not fully rotate to the unlocked position? Is it a deadbolt mortise or latch mortise. If the key is spinning continuously the tail piece on the cylinder is broke or has separated from the lock. Replace the cylinder but it will need to be rekeyed or replace the tailpiece if it came off and it is the type that screws on then you don't have to replace the cylinder or have it rekeyed. If there is internal failure in the mortise lockset, especially if it is 10 to 15 years old it should be replaced. This will normally require the services of a locksmith. If your're in Maryland let me know.