Did you also try to reset the iPhone to factory settings, to rule out, that the iOS software caused the issue?
To rule out defective data from the backup, set it up as a new device and test the front camera.
Restore your iPhone, iPad, or iPod to factory settings using a computer - Apple Support
If the front camera works after that, restore from the latest backup and test again.
In case the camera does not work after setting it up as a new device, this is a hardware issue and the iPhone has to be serviced:
Apple - Support - iPhone Repair and Service