Well, NPC's like Keldorn, Jaheira, Minsc ect. will leave if your reputation drops too much (or with evil characters, rep. is getting too high) In your case it sounds more like a bug (do you have the patch installed?) Resurrection should be done by either a priest that has the spell in his spellbook or on a scroll or find a temple. If you dropped Keldorn and want him back, I thought he is in the halls of the most noble order of the radiant heart (temple district).
The key to locate items or bodies is TAB. before the battle, cast remove fear (a cleric can do that (aerie or Anomen or Viconia), keep them alive, if you resurrect them, they should appear in your party again and with you, their equipment is there were they died (the last time of course). I personally do all the quest before entering chapter 3, If you have a higher level things will be easier when you search for Imoen.
Hope this helps. good luck. btw welcome
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There's something in your eyes, ah no, I see now, it's the reflection of my deadly gaze - Mirac Honorguard
(Necromancer)
http://www.angelfire.com/rpg2/mirac