We need a bit more info..
Do you have a Time Capsule or Airport Extreme with external disk?
Does the disk show up ok in Airport Utility?
If external drive is it formatted HFS+.. TM cannot backup to a drive in any other format.
Pondini is the expert on all things that go wrong with TM.
Look at the setup of TC or airdisk.. under gory details, here. http://pondini.org/TM/Home.html