[w00t]

The door is 20 years old and is the first time we've had this problem. Usually we can just push the door and try the key a few times until you hear the click of the lock going back, but this time it refuses to budge completely. I think I'll just take the door off its hinges and see what I can do to the locking mechanism.

Quite possible the spring is malfunctioning then. You could likely just replace it, but it would be far easier to just buy a new lock if greasing it up fails to work.

It might be easier to try to unscrew the knobs and remove the lock that way if the bolt is still engaged into the doorframe. Taking it off its hinges would require you to manipulate the entire door to get the bolt out, vs manipulating maybe a quarter-kilo chunk of metal.