It sounds like the "ring" or "collar" or whatever you want to call it may be screwed out far enough that the "head" of the key is hitting it and that's keepin the key from going all the way in. If that is correct then the only solution that comes to mind is if you have a spare key, or better yet, if you can go get a cheap brass one made at Home Creepo or Slowes and cut the "sides" off the head so that it will fit down inside the collar far enough to allow you to turn it...

__________________
1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown