Desktop computer refuses to boot

[Nuyuu]

I made a bad a month ago and I had to disassemble my GTX 660Ti GPU in order to clean up and apply a new thermal adhesive since my PC crashes whenever I use any program that uses my GPU excessively. Once I reassembled it again and put it into place with absolutely no accidents the PC just wouldn't start up anymore, I went through Google to see tips on what could be wrong but nothing was out of place, I disconnected the power button from the motherboard "pips" and used a screwdriver to try to force the boot like they do in YouTube tutorials but it didn't work.
I connected my mouse, camera and loudspeakers and they were powered so it lead me to think it is not the supply unit but then I noticed that they are all connected to the motherboard USB ports so I don't think it is the motherboard either, if it is necessary I'll post my CPU,PSU and motherboard information in a response. Thank you for your time. (this was written from a phone so sorry about the formatting too)

 

[Cycloid Torus]

REQUIREMENTS

• 450 Watt or greater power supply with a minimum of 24 Amp on the +12 volt rail.****
• PCI Express, PCI Express 2.0 or PCI Express 3.0 compliant motherboard with one graphics slot.
• Two available 6-pin PCI-E power dongles
• Windows 10 32/64bit, Windows 8 32/64bit, Windows 7 32/64bit, Windows Vista 32/64bit, Windows XP 32/64bit

Are both 6-pin connectors made? Is there any indication that they need to be split to different rails (some PSUs have single big rail, others have 2 smaller ones - having both 6-pins on same side of a 2 rail can be too much for a PSU)

 

[Nuyuu]

PSU: ATX-550W P4
MB: AM1B-M ASRock
No, this is not the first time I try to run the video card on my computer it just started melting down recently that's why I switched the adhesive, I'd like to upload images of how the setup looks but this site fails all of my uploads.
Just give me an advice, should I buy a new motherboard, new computer box or what...

 

[Cycloid Torus]

Advice: troubleshooting.... read the manuals (download from maker)
Advice: troubleshooting.... keep notes
Advice: troubleshooting.... check all connections first
Advice: troubleshooting.... make it clean
Advice: troubleshooting.... make a flowchart / decision tree with cheapest changes at beginning
Advice: troubleshooting.... replace suspect parts with known good parts until the problem goes away.. the last part you replaced would be the bad one (probably)...

GTX 660Ti GPU uses a lot of power - needs 24 amps on rail to which it is connected .. many 550w PSU have split rails, like this one.. https://www.newegg.com/Product/Product.aspx?Item=N82E16817371016 ... and only 22 amps per rail
Be sure to work out your power budget before buying parts.

Thermal Paste is not adhesive, it is a coating which fills small voids between chip and heatsink so heat can move across the space. Use a small pea sized dot in center of chip - it is spread by the force of attaching the heatsink, and why you need to tighten screws in a crisscross pattern starting with slightly tight and increasing to firm (but not forced)

 

[Nuyuu]

During the night at some point the computer had turned itself on and was stuck in a failed system restore loop.
Apparently the best solution when everything is in place is to just let it sort itself out with time...
Thanks for your support :>

 

[Cycloid Torus]

Glad to help. Sometimes with these updates, it appears there is a problem...it can take a couple of restarts to sort out.
May you recover easily.