My everyday laptop is a 2007 T61 Lenovo Thinkpad running Windows XP. Up to yesterday, it ran reasonably well given its age. I generally keep it running 24/7, and dim down the screen to the max at night.

Yesterday, I used the laptop during the day, then went out for a few hours. Nobody used it in the meantime. When I came back I found the laptop running with the screen nearly black. When I looked closely, I could kind of see that the screen was still displaying the window, but so dark it was un-usable. I could see the display change when I pressed Ctrl+Tab, but barely. I was running a few tabs on Mozilla and iTunes at the time, plus Eset NOD32 and Carbonite. The only recent change I've made is to update iTunes a few days ago.

The laptop works fine when connected to a separate monitor (I'm using it right now).

What do you think causes this problem and is there anything I can do to fix it?