Hi... lookin at your profile it appears you have a LG 5550... If your phone has worked in the past, It may have been switched to an unused NAM.
You can switch it back by doing the following.
MENU - SETTINGS - SYSTEM - SET NAM - select NAM 1.
In extremely rare cases it could be possible that your programming information reverted to factory (possibly caused by damage ~ liquid/physical) at this point you could call in customer service or take your phone into the store to go through programming on your handset..
Hope this info helps