Sounds like one of those deals where the lock isn't really unlocked even though the lever and key cylinder feel like they are working. They get gummed up with old grease and years of dust, and one day just stop working. Unfortunately, it's always in the lock mode when it happens.
Murphy's law.
Try to get a long narrow screwdriver or prybar down in there from whatever you can access, and force the lock open at the source. Youwill have to determine which way the lever moves on the lock itself, by pulling on the lock plunger.