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Can you please elaborate on the issue that you are encountering? Are you experiencing no service everywhere that you go or only in a specific location? Are both voice and data not working? What device are you currently using?
What kind of phone?
First thing I like to do when something like this happens to me is restart/reboot my phone.
On my iPhone I will;
turn off WiFi & then hold down the top button for 5 seconds and power off(keep it off 5 or 6 seconds)
On my phone with a removable battery;
turn off WiFi again and remove battery(while the phone is still on). Keep it off a few seconds and put back in.
Try those steps and let us know if it still happens.