If you've not done so already, try toggling flight mode to ensure your phone hasn't "dropped off" available coverage.
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. The wider an area or the longer a timescale that you're without coverage, tends to point towards a phone issue rather than a network one.
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.
