Try restarting (turning off and back on) the tablet itself and see if that resolves it. If not, then I would suggest going into "Settings" and then "Apps". Locate the camera app (not ones you have added) and tap it. In there clear the data and then restart the device again.
NOTE: You should back up anything associated with the camera (like photos and videos) before doing the above. Just to be careful as to not lose anything. They technically aren't part of that app, but you never know how a device will react.
Should that not help, then try starting up the tablet in "Safe Mode" and see if the camera works in there. Again, we are talking just the camera and its app that comes with the device, not any other apps. If it doesn't work in "Safe Mode" either, then either there is something on the tablet that is interfering. An app, virus, malware, etc., or the camera itself is the problem and may need repairing/replacing.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.