There are many types of undead that will "drain" your levels when they attack. You may be a level 12 fighter, but that vampire just made you into a level 10 fighter. This is easily fixed with a Restoration spell, but there are some other consequences of level drain as well. When a spellcaster is drained, they lose their higher level spells. So, to protect against this: Memorize your most important spells first. That way, they're the last spells to get level-drained. Examples of Important Spells: Lesser Restoration, Raise Dead, Heal. This makes mopping up after vampire encounters a lot less annoying. When you restore levels, spells are not re-memorized, so don't forget to do this before you rest. Negative plane protection, which protects from level drain, has a disappointingly short duration, and having to memorize and cast it 6 times before a fight is obnoxious, so I recommend that you do one of the following: 1) Cast it only on your spellcasters (re-memorizing spells is annoying) 2) Keep your casters in the rear and don't cast it at all 3) Use limited wish instead to protect your whole party with one spell. (best choice) Found on: http://db.gamefaqs.com/computer/doswin/file/baldurs_gate_ii.txt

Or use cheat codes to raise your experience points back and to pay for restoration spells :) I still haven't finished that game yet.