You need to secure the ceiling. There is a reason that it cracked. It's hard to give a great answer without pictures, but find the joists near the crack & throw a few screws in it. Then cut away the uneven parts, tape it and float it out. In an extreme case, you may need to cut out the bad section, patch it with new sheetrock & float it out. It sounds like your home may have plaster, if so, see my first response.