How do I reactivate my iPhone 4s? It won't let me after doing a hard reset.

The last few months I have been having a problem with the connection between my iPhone 4s and Sprint's service. It keeps going into "Searching...." mode. I have contacted them a few times now and have tried all of the tricks they've told me to do. They all got my phone to work again, but only for a short amount of time. Today I was told to do a backup using my icloud and then do a hard reset. I just got done doing both of those, but my phone won't activate. I keep getting a message to try using iTunes (I am trying to use iTunes). Why won't my phone activate now? What can I do to get it to work again?