Dark screen tab A

[Dinger]

On trying to open the tablet today the screen is very dark and you can barely see the numbers to put in security. Something has happened to the brightness settings which has rendered the tablet useless, you can't even see the restart if you press the power button. Any ideas please

[Dinger]

As a further I went into a dark room and managed to see enough to get into settings. The brightness was zero and was ok when sliding it up, but have no idea how this happened. Thanks