Hi Chad,
Sounds like you'll need to reinstall. But here are some things to try.
Make a backup (call it BK1) now of your UO folder (UO1). Then if you have a backup (call this BK2) of the UO folder (like from a system backup or another HD), copy the files from the backup (BK2) over the top of the actual folder (UO1). Now copy BK1 over the top of the new UO1.
No backup? Try this...
Make a backup (call it BK1) now of your UO folder (UO1). Now from wherever, reinstall the latest client you have over the top of UO1. Try running UO. If it works, cool. If it doesn't, copy what you do have in BK1 over the top of UO1... and trying running UO again.
Barring that, copy your client settings first... then reinstall fresh. Good luck Chad.