Laptop won't start!

Hello,

I opened my laptop to clean the fan and to change the thermal paste for the 5th/6th time. Now I press the power button, it starts, always keeping the screen black, and after about 15 seconds, the fan starts running at full power and the laptop turns off.
I already opened it again to see if anything was placed the wrong way, but it still not working.
Any idea?

Thanks