It's a sort of bug; certainly that part of the game is too user-unfriendly, because you only get one chance at it. It's easily done, and there's no way in-game to correct it. There are three alternatives, given in decreasing desirability:1. Do you have a suitable back-up save?
              If you don't keep an actual formal back-up save, check out the autosave.
2. If you know how to hex-edit the savegame file, you can get your healer his promotion, but you'll miss out on the experience for the quest. (The healer to priest quest is one of the two that you can't get the XP for unless you promote a character to that rank. The other is scholar to lich.)
3. Go for a druid instead.
Failing that, it's a case of start again.