I've checked the battery and rebooted but it's still dark.
It's not very old and the screen it normally too bright if anything. Now it's too dark :( I've not played with the settings unless I've changed it accidentally.
You can increase/decrease the brightness from the keyboard, you will have to read your instructions as all keyboards have a different way of doing this.
Failing that, the "light source" behind your screen may need replacing. My Acer was just out of guarantee when the same thing happened. �60 to replace including return P&P.