No signal to either screen

Hi all, first time poster, if I am in the wrong place I apologise,

My issue started about 6 weeks ago, my wife's PC was working perfectly, but it was time for a clean out and to re apply thermal paste, I done this just like the many times before, however when I went to power it back up neither screen is receiving a signal, I have went through as much troubleshooting as I can, from checking ram to resetting the cmos, I even ordered a new power supply, but still no signal, if I need to order a new GPU then so be it, and I would have done this already if I knew that it would fix the issue, has anyone ran into this issue before that could help steer me in the right direction, it is driving me crazy so any help would be greatly appreciated.

I am receiving no alerts when I power the PC up, the motherboard is fully powered and the fans are working on the PSU, GPU and CPU.

I should have added the PC parts apologies, Asus Prime Z390 motherboard, i5 9600k and a GTX 960,

Apologies for the lack of information, I used an anti static wrist band whilst cleaning out the pc and applying thermal paste, I have taken the PC fully apart and put it back together twice now (also using the anti static), and double checked everything is connected properly, the new PSU is a Corsair CX650F, the old one was an Integrator Mod XT 750w, I have tried removing the GPU and plugging it the onboard display and still get no signal, the PC is as quiet as it was before the issue, no weird noises or cables getting hit by fans etc. I cleaned and moved the ram about, tried with one ram stick (2x8gb Corsair vengeance sticks) and then the other to no avail.

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I dont know if the OS etc is useful information but it is Window 10 64 bit professional, with a 1TB HDD and a 128gb SSD if I have missed anything dont hesitate to put me in my place, thanks for taking the time to reply.

The SSD has the OS, its actually a 256G not 128G, I cant access the bios as the both screens say no signal, I havent tried booting from a USB, so I will try this, I dont know how to do it but I am sure Youtube will have a video for it, the PC doesnt have a DVD drive, I have tried removing the GPU and plugging the screen into the motherboard and it still gave no signal, both screens work on my daughters pc fine, I will go and order a new CMOS battery, the PC is about 4-5 years old.

Would it matter if it was an AMD motherboard I tried the GPU out on? my daughters PC is AMD and this is one of the things I wanted to try but didnt know if I could.