Question New build doesn't power up

[LeChatQuiRit]

New build all new components. Everything is connected correctly and double checked, but the PC doesn't start! I turn the switch on the PSU and the logo on the MB lights up, so I know there's power coming into the MB. But when I push the power on switch -- nothing happens! No signs of life at all! Not a fan stirs!
This isn't my first build, so I felt relatively confident while building it. And so it's a surprise that nothing works.
I've tested the PSU with a paperclip -- it works.
Yesterday I removed the MB from the case and put it on the cardboard box. Reconnected all the cables and pushed the power button. It worked. Today I re-seated the MB, reconnected everything,... and it doesn't work! I took it out of the case once again, connected the power cables for the MB and the 2 8-pin connectors for the CPU, re-seated the graphics card and connected both PCIe cables to the graphics card (one 8-pin and one 6-pin). Pushing the power button -- nothing! No life! Only the LED logo continues to change colours. But no fan stirs, no LEDs light up -- nothing!
I thought it might be the memory, so I tried with each of the two RAM sticks in every one of the 4 DIMM slots -- no change.

Specs:

• Motherboard: ASUS Prime X670E-Pro WiFi
• CPU: AMD Ryzen 7 7800X3D
• CPU Cooler: Arctic Liquid Freezer II 280
• Memory: G.Skill Trident Z5 Neo DDR5-6000 C30 DC - 32GB (2x16GB)
• Graphics: Sapphire Radeon RX 6700 XT Pulse - 12GB GDDR6 RAM
• PSU: Corsair RM850x 850W PSU RF
• Storage: Kingston FURY Renegade PCIe M.2 NVME SSD 2TB
• Case: Fractal Design Define 7 Mid Tower

What else can I try? Is there a way to test the CPU itself? The Q-Led diagnostic on the MB is supposed to light up and indicate if the CPU is faulty, or if the RAM is broken. But they don't!
I can provide pictures if they'll help.

 

[punkncat]

When you mention breadboarding the hardware and "pressing the power button", which one was that?
I didn't specifically look up your motherboard, but know that some have an actual onboard switch.

Leave in case, remove front panel power connector and see if you short the pins = turn on?

Check backplate of case for extra standoff not being used grounding on something?

Was there any difference in the system out of the case as opposed to back in the case such as drives or graphics card and so forth?

 

[LeChatQuiRit]

When you mention breadboarding the hardware and "pressing the power button", which one was that?
I didn't specifically look up your motherboard, but know that some have an actual onboard switch.

Leave in case, remove front panel power connector and see if you short the pins = turn on?

Check backplate of case for extra standoff not being used grounding on something?

Was there any difference in the system out of the case as opposed to back in the case such as drives or graphics card and so forth?

There is a power on button on the motherboard itself.
The case has standoffs for ATX boards. I can't see behind the board when it's mounted, but it doesn't appear to be the standoffs.
When I first tried breadboarding and it worked, I mounted the MB back in the case and connected everything -- SSD, the fans, the Front Panel USBs, Front Panel connectors, etc. When it refused to power up, and I took it out again, I disconnected everything except the essentials: the 24-pin ATX power cable, the two 8-pin CPU power cables, the cooler lead that goes into the CPU an connector, the graphics card, and the two PCIe power cables that go into the graphics card. This second breadboarding has the same connections as the first one, yet at the first breadboarding it powered up, and now it won't!
I don't know what can be the problem. Could I have damaged something while mounting the MB into the case or while taking it out again? PCIe slot? The graphics card can move a little up and down, but it seats firmly in its slot.

 

[Pc_idiot]

Build:

Asus x670e gaming
7800x3d
Sapphire pulse Radeon 7900xtx
Lian li TL fans
Corsair rm1000x
G.skill trident x5 RAM
LIAN LI aio

So what ended up happening? I’m in the exact same boat and I’m at a loss for what to do next.

 

[LeChatQuiRit]

Build:

Asus x670e gaming
7800x3d
Sapphire pulse Radeon 7900xtx
Lian li TL fans
Corsair rm1000x
G.skill trident x5 RAM
LIAN LI aio

There were two issues with my rig.
The first one was the BIOS. The version that the MB came with had some problems with recognizing memory: only one of the 4 slots worked. So I flashed the BIOS to the latest version (thankfully, the MB had this facility).
The second issue arose in the process of building and connecting the components. The main cable from the PSU to the MB (the ATX 24-pin cable) broke. It fed power to the MB if you pushed it a little to the side, but as soon as the pressure was removed the MB turned off. So I had to buy a new Corsair ATX 24-pin cable.
When both issues were corrected it started working immediately.