No signal on 4S. Carrier Shows.

Hi Guys, this is my first post here. And that means that ive tried alot before i reached out to the internet to fix this thing..

Ive bought a phone second hand, but with the receipt and everything.
Its an iphone 4s, which is simlock free, so i used my own prepaid card.
Everything worked smoothly for a month, but since yesterday i have no signal anymore. The carier shows, and i got wifi. but no signal bars.

I tried everything, from resetting network settings, to reinstalling software. Nothing worked. Could it be reported stolen maybe>?

Or is it a common problem ?

Any suggestions appreciated !

Here is an example pic that shows my problem.