Screen suddenly dim on Samsung N220

Overnight the screen on my Samsung N220 is very dim.

The Samsung hardware key combinations for brightness appear to work, but don't: little brightness-adjustment graphics appear on the screen, however, the screen does not get any brighter as a result.

I installed xbacklight, no effect.

I assume this is a result of automatic upgrades I made yesterday. The version of Linux is

Kernel : Linux 3.2.0-51-generic (i686)
Compiled : #77-Ubuntu SMP Wed Jul 24 20:21:10 UTC 2013
Distribution : Ubuntu 12.04.2 LTS

I would be very grateful to know what to do with this - I don't mind researching and learning about Linux for a major result but it is boring to be trying dozens of different things for such a trivial issue!

Cheers

Andy