My Laptop was switching off but now when i turn the laptop on it makes the sound as if it is looking for a boot sorce but the screen stayes blank.

Please follow the diagnosis outlined in this article to check whether the backlight lamp or the inverter board are the root cause of the problem or not:
http://www.fixya.com/support/r6430776-troubleshooting_laptop_power_failures

If you connect the external monitor & nothing is displayed on it, then this means that both the backlight lamp & the inverter board are fine. The main indicators in these cases are the sound, and the hard drive LED on the laptop case. If the laptop sounds just like it always does for a minute or two after being turned on, it means it's going ahead and booting the operating system, but the video processor or part of the motherboard circuit for video output has failed. If the laptop just sits there and beeps when you turn it on, or if you hear the fan but not the hard drive chuckling with its active LED flashing away, it means the laptop isn't getting as far as trying to boot the operating system. While this could be due to any number of hardware problems, the most common and easiest to
troubleshoot or repair is RAM failure. If your laptop has two RAM modules installed, try booting with just one and then the other in the primary slot. If your laptop only has one RAM module, you might consider buying one online from a site with a good memory selector, like Crucial.com.