I got the code from cingular and i want to put it in. But the subsidy code screen doesn't pop up anymore because i put in the wrong code too much. It just say Contact Service Provider. How do i get the screen back?

Depends on the make and model of the phone. Most makes of phones, if the code is entered wrong to many times the phone will become hard locked to the origanal carrier (att)
As example, Nokia phones (depending on model) will give you 3 to 5 trys and if entered wrong will hardlock the phone
Motorola (some models will hard lock after too many trys) others, by shutting off the phone for a couple of hours will bring back that screen.
Again, it varys from make and model but chances are, you have hardlocked your phone and the only way to unlock it now is to bring it to a shop or mail it in for unlocking