Dell D620 Boot problem

Hello everybody,

Background:

I have an old Dell D620 laptop that has been with me for a very long time, since 2006. Until last week, there was no serious problem with the laptop except that the fan was very loud. In fact, it was so loud that sometimes I would stop using it in public places like library or starbucks in order to not bother others.

Over the years, I have replaced and fixed numerous parts of this laptop, including memories, hard drives, LCD screen and its hinges, bluetooth chip, etc. But I never faced the problem I encountered last weekend.

Problem:

I opened the laptop completely. took the fan and heat sink. Not surprisingly, there was a clot of dust blocking the path between the fan and duct. I easily got rid of all the dust in and around the fan. Next, I cleaned and removed the old thermal paste and replaced it with new paste. So far, no problem! I put the laptop back together and everything seemed fine. BUT, when I hit the start button, the laptop turned on, the green battery light turned on, the hard drive started spinning, and...then the laptop shut down! it didn't stay on for more than few seconds. The screen never turned on either.

So, I started with the basic diagnostics that I know.

I started by checking the connections between the LCD, keyboard, and everything that connects to the motherboard. Everything looked fine!

I, then, tested the memories individually, and in each slot separately. Unfortunately, that didn't help neither.

Now, I am stuck and don't know how to solve this problem. My laptop is old but it has been very good to me. Have you encountered this problem? Does this sound like a familiar problem? And do you speculate that I killed the motherboard somehow while cleaning it?

Thanks a lot.