If you suddenly cannot lock your Milner safe with it’s key, I made this video to explain a common error.
Milner safes with the chevron keyway, also old Ratner safes and some Whitfield too. The key only goes into the same position that it came out of. Normally when locked, you put the key in at the 7 o’clock position, turn clockwise to unlock. If you usually leave the key in while the door is open, then you only ever use the 7 o’clock slot. If one day someone unlocks the safe and takes the key out, then sometimes people try to put it back in at 7 o’clock to lock. All you need to do is put the key in with the bit (aka flag) pointing up to 11 o’clock and it will go in further and lock normally again.