This issue is common among BlackBerry Torch 9800, Bold 9780 and Curve 9300 users. You see SOS instead of your signal bars. Poor signal reception is the most frequent cause of this issue but the SOS sign might remain even after signal is available. In some cases, it's a faulty radio hardware.
To get signal back, you can try the following:
1. Turn off mobile networks then turn back on
2. Do a battery pull
3. Update the Blackberry's OS (see http://blog.hovatek.com/how-to-downgrade...d-version/ )
If this issue persists even in an area with strong reception despite an OS update then you might have a hardware issue on your hands