My laptop will not boot after I opened it up and cleaned the heatsink

I had a problem with my Dell Smart PC 250N of overheating and shutting off after half an hour or so. I searched over the net and as per instructions on a web site opened up the laptop, cleaned the heatsink and the CPU and applied arctic silver thermal grease to the CPU and reassembled everything, thinking that everything would be fine now! To my horror now the laptop would not boot up!

As far as I am aware I have not damaged anything knowingly, the CPU seems to fit in the slots well and have tightened the heat sink well.

What do you think has gone wrong and what is the solution? If it is a goner then how can I retrieve the tons of files from my hard drive that I foolishly did not back up before trying this stunt.

In order to boot any laptop with video you need three main components: motherboard (with video card), CPU and memory and they have to be properly connected to each other.
Make sure the memory module is making good contact with the motherboard. Try reseating the memory module.
Make sure the CPU is making good contact with the socket. It’s possible that you accidentally pulled the CPU from the socket while removing the heatsink. Remove the heatsink again, unlock the socket, reseat the CPU and LOCK the socket. Also, when the CPU is removed, take a closer look on the pins. Hopefully the pins are not damaged.

If all three components are in good working condition AND properly connected to each other, you should be able to boot the laptop, at least to the Dell logo.

Just in case, you can transfer data using an external USB enclosure.

Do you need spare parts for your laptop? Search here by the laptop model and part description.