Question Computer Lights/fans turn on. Monitor shows no display. Keyboard and mouse have no power.

[Chonkyv]

Recently upgraded some of the specs on my PC. After completing the build, the computer powers on, LEDs from the fans and gpu come on, but my displays show no signal and the peripherals don't come on either. I've tried a few things I've found on forums but nothing has worked so far.

My Specs:
Windows 10
CPU: intel core i3 8100
GPU: RTX 3070 Ti FTW3
MOBO: Gigabyte H310M A
PSU: Corsair CX750M
RAM: Oloy 16gb 1.35V 3000MHz

Things I have tried:
Initially I had an i59400f cpu installed. Had the same issue and thought maybe I had to flash the BIOS first, so I swapped to the i3 8100. Still having the same issue with the i3 8100.

Tried swapping the ram for the old 4gb sticks I had, no dice
Tried replacing CMOS battery, no dice
Tried starting the computer with 1 ram stick at a time. Didn't work with either.

I don't know what else to try. Id appreciate any suggestions, thanks.

 

[Ralston18]

PSU: How old? Condition (original to build, new to build, refurbished, used)?

History of heavy use for gaming, video editing, or even mining?

This motherboard:

https://download.gigabyte.com/FileList/Manual/mb_manual_h310m-a-20_e.pdf

[Do verify that I found the applicable manual.]

Power off, unplug, open the case:

Referencing the User Manual double check that all internal connectors, cards, RAM, and jumpers are correctly placed plus fully and firmly in place.

Carefully disconnect and reconnect (or unseat and reseat) each component a couple of times to be certain.

Leave the CPU alone.

 

[Chonkyv]

I should add that the parts I replaced recently were the CPU, Motherboard, and RAM. Everything was working before upgrading these parts.

This looks like the correct user manual

PSU was purchased new in 2014, so it is fairly old.
Gaming is the most intense thing I use the computer for. Just tried reseating GPU and RAM but no dice

 

[Chonkyv]

So I have some more information. After trying to reseat everything once more, I decided to reconnect my old motherboard and CPU, but left the new RAM installed. Everything works, keyboard lights up and i have a display on my monitors. So that eliminates the RAM or the PSU right? Is there something I'm maybe missing or did I just get a bad mobo?

Old Mobo: MSI B250M Pro-vd
Old cpu: intel core i5 7600k