what you have means what you are using WIndow or Mac. if you are using Mac then have few steps for checking what is the exactly problem is. connect your phone in Mac:
1.Open system Profiler in Utilities.
2.Click on Usb option in left
3.Select iPhone and see the serial number over there.
4.If have fffffffffffffffffff like this then your phone's IMEI is corrupted.
5.Visit the Service provider as soon as possible, they will replace this phone will brand new iPhone.
6. If have values like :
Product ID: 0x12a8
Vendor ID: 0x05ac (Apple Inc.)
Serial Number: 66464794b46b6a12e98d405fbd34475
Speed: Up to 480 Mb/sec
Manufacturer: Apple Inc.
Location ID: 0xfa140000 / 6
Current Available (mA): 500
Current Required (mA): 500
Extra Operating Current (mA): 500
showing some other problem.
7. For windows there are softwares like USB view. In this you can find out the same info about the phone .
First check this then msg me .