Monitor is too bright… Brightness cant be controlled!!!

Hi guys! I’m using a Philips CRT monitor. When I was working on the computer, all of sudden the monitor brightness got changed and now the screen is too bright. The brightness can’t be control with the front side controller!! However the screen contrast can be changed! . After I got this problem, I saw a very bright light appears in the middle of the screen when the monitor is power off. ( just like a camera flash light) .; can you suggest what can be the problem ?

The monitor symmetric diagram is attached herewith. (Monitor Philips 104S.pdf - 4shared.com - document sharing - download)

Monitor : Philips 104E