If a character is level drained by an undead in AD&D (and the Gold-Box games), they lose a level or two for each time they are afflicted (depending on the undead type that hits them) and their XP is reset to the beginning of whatever their new level becomes, I think. They can regain it normally, or they can visit a temple to have a Restoration spell cast upon them. In higher level games like "Pools of Darkness," Clerics in the party can eventually cast it themselves, but at low levels you have to pay someone to do it or find a scroll with the spell (it's a 7th level Cleric spell, so not easy to find). I seem to recall there being a "Lesser Restoration" spell in AD&D, but I don't recall if it exists in the Gold-Box games and I can't find any evidence that it does.
After thinking about it, I came up with another possibility of what could have happened:
If the character was ready to level up and then also got enough XP to level up again, they would stop gaining XP.
Example: A 1st level fighter needs 2,001 XP to reach level 2 and 4,001 XP to get to level 3. If a fighter were to hit enough XP to reach level 3 while they were still level 1, it would cap them at 1 point less than what they needed...so XP would be stuck at 4,000 and all XP gained would be lost. This is to prevent the character from gaining more than one level at a time. This might have been a rule in AD&D, but I can't find any indication of it at the moment (doesn't mean it doesn't exist, though).
I have seen an odd error occur where a character could get stuck because they need to level up in one of their multiclass levels, but the game prevents it for some reason and then the progression stops. Could be due to a mismatch in the code where the game thinks the character can level up but then the training hall correctly states they can't. This is all a theory, though; because I only saw it happen once (also with a Cleric/Fighter/Magic-User), but I don't remember which of the Gold-Box games I was playing when it happened to me (though I was not playing on an Amiga).
Anyway, something to think about.