Looking at the back of the phone, press in the upper right hand corner and hold it there, see if you can get a signal. A quick simple fix for this without opening the device can be to get a plastic hard case and something as simple as a puzzle piece, insert puzzle piece into upper right corner of case, ensure doesn't block camera, and insert phone, see if this doesn't help. Sound weird? Here's proof http://www.youtube.com/watch?v=z8Ha8tkiJ-w
Other than that I would suggest opening up the phone and ensure all cables are connected and seated correctly