Hi. This may result from one of a number of factors
- No SIM card inserted or detected - try a different SIM or restart the phone
- The phone is on flight mode - some phones have a flight mode feature that can be toggled on and off (we will need to know your phone's model to advise much on this)
- Phone is blacklisted by carriers - if a phone was reported lost/stolen, carriers blacklist it s IMEI number to keep is from connecting to any of the national carriers until found.
Kindly comment below with a bit more info on this phone and we will gladly advise further.