Welcome to Windows Central.
You may try the following:
1. Restart your phone.
2. Perform a soft reset.
3. Perform a hard reset.
4. Make sure you have a SIM Card inserted, and that the SIM contact points are clean.
5. Make sure that you are in an area that has cellular coverage. Are the other phones on the same network in your area working?
6. Try inserting your SIM Card into a different phone.
7. If all of those don't work, have your phone replaced (if it is still under the replacement period), or repaired.
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