When i push power all the fans go on, but the monitor stays black.

I recently put in a new motherboard (and PSU), trying to fix it not booting at all. Before that everything was working perfectly for 5 years.

The only difference with this new motherboard is instead of there being an 8 pin CPU power connector on the motherboard, there is a 4-pin labeled ATX_12V. I don't have a cable from the PSU that fits it, I tried booting without it and with half of the 8-pin cpu cable, but STILL no GFX, no beep codes, nothing. The fans just keep spinning and the monitor keeps doing nothing.

Motherboard: Gigabyte 970A-DS3P
GFX: GIGABYTE GTX 770
RAM: 3 x DDR3
PSU: corsair cs750 (new)

starting to lose hope.
If the card needs ab 8pin cable you need a PSU with an 8 pin cable...

BTW THATS a PSU that i would not choose...Are you sure? Do I seriously have to buy yet another PSU? I just bought one to try to fix this only to discover it wasn't the PSU. It's a modular one so maybe i can just GET a cable?
I bought that PSU because it's cheap, I'm not trying to build the best computer, I'm just trying to get this to run.

Also, it's not the card, it's the motherboard, and it doesn't need an 8-pin cable, it needs 4-pin (I assume).In this case there is no problem using 1/2 the 8 pin connector for the ATX 12v. The motherboard is getting older was it new ?

Also is the CPU on the motherboard list here is the list

https://www.gigabyte.com/Motherboard/GA-970A-DS3P-rev-2x#support-cpu

Also the old motherboard not booting at all how was that diagnosed to the motherboard being faulty? Could it be a faulty CPU ?

Also can you check the SPECS on the memory as it shouldn't be low power.