Screen turns black when put of electricity

When I put my eee 900A on battery power, so I take the electricity cable of the computer, the screen turns black. What can I do about this?
The battery was loaded at about 30% so that shouldn't be a problem.
It isn't shut down because the lights burn and if I put the cable in again, I see everything again but also a sun loading.
I tried searching for energy options and see if it automatically goes to sleep or something, but that doesn't seem to help.
Anyone knows how to solve this?