If it's the original door lock than it's probably just stuck. That happened on my '98 driver's door a few years back. I just sprayed a little penetrating teflon dry-lube into the keyhole and cylinder and then worked the key around. Freed it up in just a few seconds, and I haven't had a problem with it since. It's a good idea to lube the other side while you are at it.
FWIW, I have found that nothing beats PB Blaster's Teflon Dry Lube for things like locks, hinges and other things, such as vinyl window tracks, for instance. Places where you really don't want an oil-based lubricant, for whatever reason.
'98 Ranger XLT 4x4 (4.0L)
'97 Taurus GL (3.0L Vulcan)
'94 Taurus SHO (MTX)