what do i do if a quest is broken??

how do i get by it

abandon it and move on

Depends on the quest. If the quest is important (IE: a class quest) you can spawn the item in your inventory (.lookupitem <desc>;.additem <ItemID>) if it's a beast you have to slay, you could try creating a spawn for it (I haven't tried that, but it seems like it would work..) if it's not an important quest, just skip. :)