Try swapping SIM cards with a known working, same network phone and see if the problem stays with the same phone or moves to the other phone.
If it moves you either have a faulty SIM card or you need to check with the service provider whether the account has been blocked for some reason. Lack of credit/non payment of account perhaps.
If it stays then there is a problem with the phone. Try a factory restore of the phone to see if this resolves the problem. Backup the phone first as a restore will erase all the user data and downloaded apps.
If the factory restore fails to resolve the problem there is a hardware fault with the phone.
If the factory restore (also known as a hard reset) resolves the problem, restore the phone using the backup made earlier and see if it still works OK.
If it now fails again a downloaded app may the cause of the problem. Uninstall the downloaded apps one by one testing the phone between each uninstall to see if it works. When it works the last app uninstalled will be the culprit.