Easy!
First, make sure all applications are closed and rename your drive to something unique so this doesn't happen again.
Next, open rekordbox and go to your collection. Select all (CTRL+A on Windows, Command+A on OS X) and right-click on any of the missing files. Select "Relocate."
The first track will ask you to point to where the file lives - simply browse the folders to that new location, select the file, and because the only change in location for your files was the drive name, it will automatically relocate all your other tracks.
Please let us know if you have any issues with this process.