If they removed one of the bolts from the top of the selector box on the top of the gearbox thinking it was the filler plug, they will have upset the reverse gear interlock that stops the gear leaver from accidently shifting into reverse when shifting down from 5th.
I had the same problem when I changed my gear cables and took out this bolt thinking it was one of the fixings holding the selector box in place. The gear stick wouldn't go into reverse and would get stuck in 5th (or something like that!). To fix it I had to take the selector box off and partially dismantle it in order to rotate the spring loaded cam into the correct possition. I seem to remember there was only 1 other way the cam could sit.
I doubt low oil would cause this problem, someone posted on here that they changed their gearbox oil only to find that there was only about 200ml in there but it still worked, probably not the best for ware though!
I hope this is the fix you need as it's pretty easy to do, almost a easy as the mistake of removing that bolt!
Good Luck
Knowledge is realizing that the street is one-way, wisdom is looking both directions anyway