My MacBook Pro 13'' Early 2011 is not booting.

Hello,

I have a Macbook Pro 13'' Early 2011. Last week I tried to replace the thermal paste on the processor and on the graphics chip. I followed a video tutorial and an iFixit tutorial step by step during the entire process. Once I was finished I decided to turn on my Macbook and give it a spin. Unfortunately it didn't work, although I haven't touched or damaged anything on the logic board, as far as I'm concerned.

The scenario goes as follows:

• I press the start button..
• The fan spins once, the hard drive reads for a second and the sleep button flashes for a moment. I am not hearing any chime, neither does my screen go on.
• The Macbook Pro turns off again automatically.

I am going to open it once more before it give it to the authorized Apple service. Do you have any idea where the issue might come from?

Thanks for your replies!