Like any lock, mortise locks have their common glitches and stumbling blocks, especially those that have been used heavily. The handle or knob have been known to even fall right off! In this case your spindle probably needs replacing. It’s likely that the current spindle has been worn down from use, and it no longer screws as well as it used to.
Sometimes the key fails to work on old mortise locks. In this case, the cam of the lock might be broken and in need of replacement. You may as well replace the entire cylinder in this case, as extensive wear in one area probably means rough wear in another. Still, the cam is probably the direct cause of this issue. If you’re not so sure, you’ll need to open the lock body. To do this, you’ll first need to remove the latch cover. This is likely being held down by a few screws. Be careful – opening the latch cover can cause the components inside to spring out – sometimes right into your face! Be sure to take a picture of the inner mechanisms of a mortise when you open it. You’ll be able to put it back together after you take it apart for servicing.
Another reason why your key won’t work could be that the lever is sagging. Your spring may be weak or damaged. You should closely consider the largest spring, located against the hub of the spindle. If the spring isn’t vertical, it’s probably malfunctioning, causing the key to not work.