HP ProBook 4540s display backlight does not power on?

In my hp 4540s Probook laptop sometimes the screen backlight does not turn on. But pictures are there on the screen when i see it from different angles. And external monitors work fine.

This happens when,

*the display goes to sleep and come back again later, the backlight does not turn on

*starting the computer (when powering up, the backlight is already not there)

(in both the above cases pictures are there on the screen without the backlight)

When this happens, i can get the backlight by moving a magnet several times near the right speaker of the laptop (where the lid switch sensor is, as i know).