For a mortice lock that will not open even when the key will turn I think you will need to cut the lock with an angle grinder using a thin metal cutting blade please also use gloves and eye protection

Hi do not drill the lock as you will not know the drill points and if you can't see the bolt you won't be able to use a angle grinder do you have a spare key first as it might be something wrong with the key you are using do you know what the name of the lock

something has gone radically wrong inside the lock,a locksmith could probably get it to open but the workings of the inside of a Mortice lock will be unknown to you ,as stated you cant just 'drill it' you need to know where to drill I assume you are managing to get in the house ,if so I would do as stated above and grind through the bolt,the answer above is confusing the type of lock, there is no cylinder on a mortice lock

Sounds like a piece called a curtain or bolt thrower has worn . check another key as mortice keys do get damaged over the years .if you drill make sure you're in the right place or you will be in trouble .good luck

you can try drilling the lock cylinder to see if that will release it, but make sure you have a replacement purchased first. You will need to measure from the edge of the door to the middle of the lock cylinder ( where the key goes in) . This will give you the backset size that you will need. And i would try and get a similar style of lock if possible.
If you manage to release the lock by drilling then you should just need a new cylinder unless you are planning to change the lock of course.
If drilling does not work and you cannot get anything in to release the lock then you might need to cut the staple (the part that sits in the frame) or try and prise the lock assembly away from the door. Hope this helps