One possibility to consider, and may not be the cause
An operating system update can also cause this
If was previously on another network then unlocked the update can cause it to revert to being locked to the original network. It happened to a phone I have. You will need to use the Network Unlock Code, NUC. Luckily I had kept a copy.
To find the networks unlocking policy GOOGLE the network name followed by the word UNLOCK
You will need your pones IMEI number which will be on the original box, or under the battery if it can be removed
You can find your IMEI in the following ways: by entering *#06# on your phone's call screen.