So last night I was bored (internet connection went dead) and I thought that cleaning the cpu cooler might help me kill the time. I unplugged all the cables took the computer in my bed and started demounting the cpu cooler.
I opened the little clips that where holding it to the motherboard and I unplugged it. After I cleaned it I put it back into it's slots and started the computer.
It worked until the login screen then shut down itself. I thought it was from the psu but apparently I didn't fix the slots well so that's why it wasn't running. I fixed everything and it worked fine.
But today sometimes I keep hearing the next sounds: "beeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeep", "booooooooooooooooooooooooooooooooooooop", "biiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiip" coming from the fan.
My noob head thought at the airflow making the sound now that it's path is clear (the amounts of dust I pulled out is amazing).

PS: No, I didn't use a wet paintbrush, I used a regular dry one (it was fluffy).

Apparently, this only concerns Fluffy.

I hope you didn't do anything like this.

whoa wtf!? what happened there?

Looks like someone was a bit eager with the thermal paste.

Back on topic please.

I'm not a hardware guru but either you've not connected all your power connectors correctly (maybe your GFX card, if that has seperate power), or your PSU is goosed.

Are they possibly the beeps from your motherboard that indicate something is wrong? Find your motherboard manual maybe - (totally untrustworthy advice btw, but will do as a stopgap til someone more knowledgable comes along)

You probably have remounted the heatsink without cleaning off and reapplying thermal paste. And then possibly secured the heatsink down wrong so that it is not connecting properly with the cpu.

I didn;t reapply the termal paste indeed. I will soon after I buy a new cooler.