Anumber of things can go wrong with doors: hinges, handles, doorbells, and even the doors themselves (hollow-core interior doors are particularly vulnerable to having holes knocked through them). Most of these problems have straightforward repairs that can easily be managed in half a day or less. They don't even require expensive or extensive tool lists. Screwdrivers will be your main ally, with a chisel, hammer, and putty knife useful as reinforcements.