Well, if the file is REALLY missing, then there is no choice but to reinstall Unreal Tournament. That's an essential file for the game to run I would imagine.
Other alternatives might include running scandisk.. If you've had any unexpected shutdowns in between the time it used to work and now, then the file could be corrupted or crosslinked or God only knows what else. But perhaps Scandisk/Chkdsk can fix it. If you are using Windows 98/Me, you can find Scandisk in Start/Program Files/Accessories/System Tools. If you are using 2000 or XP, then you should right click on your drive in My Computer and choose "Properties".. When the box comes up, select "Tools" and "Check disk for errors"
As a final note, your Unreal Tournament.exe should be in the "UnrealTournament\System" folder.. If it has somehow decided to pack its bags and leave its directory, then copy it back there.
Any cracks or patches that have been applied to it might have screwed it up too.