Frames damaged by a bent frame must be fixed by a certified mechanic with expertise in collision repairs and automotive bodywork. The procedure usually involves assessing the extent of the damage, removing weak spots, and placing new pieces of the frame.
The type of warranty available depends of the shop that you choose. Be sure to check what warranties they offer before committing to any repair. Most reliable auto body shops offer their customers an assurance that covers all work and materials used for a certain period of time.