Where are you? The first step is to look around and see if you're in one of those places that get worse moving waves. It may be a tunnel on the road, the elevator, the subway or a place lost in the mountains. If the return to a "normal" place problems persist, follow these steps.
It is now going by making small incremental reboots. Start with the easiest: putting your phone in airplane mode (it was not, is not it was in airplane mode if that was the problem?) For a few seconds, and then turn it off. In many cases it is sufficient to return coverage.
If the airplane mode trick did not work, restart the phone completely. Wait a bit when it is on, as sometimes the system takes to regain service.
Is everything the same? Turn off your phone, open it and removing and inserting the SIM card. Turn it on and test.
If none of this worked-and before you had coverage normally is not a configuration problem - call your carrier service or take the phone to an official store. They should find and fix the problem causing your phone to retrieve the coverage.