My wizard level 24, Katherine BattleBlood, lost the main quest line. I have looked through my quest book and the quest-line page on the wiki, and only seem to have side quests. Any help would be appreciated.
It may be possible that you accidentally declined a main quest after finishing another one. It's rare, but sometimes when receiving the new quest, it won't force you to accept it; alternatively, your client may have crashed or otherwise forced you out of the game while you were in the middle of the NPC quest-giver dialogue, causing it to not be accepted.
To remedy this, you may try using the quest finder to find the NPC who is supposed to give you the new quest, though this could just lead you to more side-quests. You could also try using the quest finder in the area where your previous main quest was, as the NPC quest-giver is likely close to that area.