If you're sure that nothing (cover or screen protector) is obstructing (interferring with) your front camera, try a "forced" restart using these instructions (to check if it is a temporary glitch):--> Force restart iPhone – Apple Support
If the issue persists, contact Apple Support directly, using this link:--> Get Support , or make an appointment and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector
After checking it out, they'll give you all the available options.
Regards
Giulio