If alternate cameras do not work, then try a Hard Reset:
Power off the phone > hold Volume Down > press Power > keep holding Volume Down until you see the exclamation (
!) screen, then immediately release the Volume Down
Now tap Volume Up / Volume Down / Power / Volume Down
You should see the spinning gears and your phone will reflash the last OS version held in recovery. You do loose data, but get your phone back. Sign into your Microsoft Account and restore your backup during the set up.
Test camera again. If it does not work, the Hard Reset ruled out 99% that it may be software. Seek repair or warranty replacement.
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