SOS appears top of phone

SOS is appearing because your phone has lost its connection to your carrier's cellular network.

Temporarily, you need to use another phone to contact your carrier and ask them to restore service to your phone.

As Bob Timmons said, it may because you lost connect to your carrier or it might be that you have no SIM card in your phone. Make sure to check.

If there were No SIM card in the phone, "NO SIM" would appear in the upper left corner of the screen.