Official comment
Sorry Simon, without a proper backup from your computer, the only thing you really have is tracks on the export device (your USB stick). You'll need to copy them back over to the computer and rebuild the beat grids and cue points.
Browse into the stick, find the music, copy it back into a folder structure you're happy with (or use what's on the stick in its crazy sorted format) then add that music to your rekordbox library and start anew. You'll also need to re-export everything as rekordbox won't recognize your external device as being exported from itself.