Blank screen on boot

Hello,
blank screen on booting
my notebook is HP Pavilion dv9700t

3-4 day ago display went off suddenly when i was watching movie on gomplayer. i had to switch off notevbook force fully by pressing power button

No display is seen after turning on the Notebook.Only blank screen with very low blue light appears on the screen. System seems to process .

following are the troubleshooting steps which i have performed.
1. I had a chat session with with online technician and and he advised to remove battery and AC power and pressing power button on laptop. but it did not work.

2. I went to HP authorized service center , progressive pvt. ltd. ,Bhopal , India. They diagnosed the laptop at INR 562 by checking each component on opening laptop parts and reassembled each parts. after diagnosing they referred it to be motherboard problem.

3. Then they tried once more to solve the problem. this time they inserted a bootable flashdrive in notebook and then they turned on the notebook. after 2 or more such attempts laptop started to work as before. screen was displaying BIOS and then windows vista booted normally.

4. But after 1-2 hours, display went off again leaving totally black screen ,while i was working on notebook. 5. I went to service center again and they did the same thing using flashdrive . they called this process as " running UTILITY". but again after some time display went off(totally black screen).

5. then they ran UTILITY again and performed the system recovery on my request.

6. and again display went off suddenly while running notebook.