In my family room I have two switches that control the ceiling light. One as you come down stairs, the other at the door as you come in from the backyard.

Recently this is the scenario...

At the door the switch is operating normally. Flip it up or "on" and the lights come on. Flip it down or "off" and the lights go out.

If the lights are on with the door switch, the stair switch when flipped up or "on" (as this has always been the way it is) turns the lights out.

If the lights are out at the door switch, the stair switch does nothing at all.

Any ideas? Thanks in advance for the help!

One of the switches is bad... replacing both is the simplest thing to do. Or you could replace just one, see if it fixes the problem. If so, fine, if not, put the old one back in and replace the other. I would replace both and be done with it. Take care to put the wires on the correct terminals by the switch markings, not just placement.