If you've not done so already, try a manual network search to see if EE shows as available.
Use the "check service status" option on https://ee.co.uk/help/mobile-coverage-checker in the first instance, to check for local service outages. The "report a problem" link should be used if no issues are reported but EE users continue to have issues.A loss of coverage will remove the ability to make/receive calls, send/receive texts or use mobile data.
If you're in the UK and have a WiFi connection, you can use WiFi-calling to keep using your phone as normal without needing network coverage.
