Laptop XPS17 failing to turn on.

http://en.community.dell.com/cfs-file.ashx/__key/communityserver-discussions-components-files/3518/3107.xps_5F00_boot_5F00_issue.mp4I own a Dell XPS 17 (L702X) which is around 1.5 years old.
One night after watching a movie, i turned it off(Hibernate). It never turned on again.

I tried dell's technical support, and they said that my motherboard has short-circuited and i need to replace it.

When i asked, they suggested the following possible causes:

1. Bad earthing(ground)

2. Water damage

3. Over heating

I am quite sure about 1st and 2nd. I got my earthing checked and neither can it be water-damage.

My laptop does generate a lot of heat while gaming, and i always use an external cooling pad, with three fans positioned right at the input vents. I had never experienced any lag or performance issue with the heat.(http://www.dell.com/support/troubleshooting/in/en/indhs1/KCS/KcsArticles/ArticleView?docid=279891)

Also, i was just doing simple stuff the last time i used my laptop. No signs of any failure of even possible failures.

It just didn't start the next time.

Observations:
1. POST is completing.

2. Unable to reach till BIOS options. System turning off within 0.5 sec after the DELL logo appears.

Please help. Even if i replace the motherboard, i want to be sure of the root cause, as it's quite expensive.