My son's new PC doesn't start - is the motherboard broken?

My son finally used his first self-earned money to buy a PC (components) and today we built his new machine. Here are the specs:

* AMD Ryzen 5 5600X 6x 3.70GHz So.AM4 BOX
* 12GB Gainward GeForce RTX 3060 Pegasus DDR6 (Retail)
* MSI B550-A Pro AMD B550 So.AM4 Dual Channel DDR4 ATX Retail
* 650 Watt be quiet! Pure Power 11 FM Modular 80+ Gold
* 32GB (2x 16384MB) G.Skill Aegis DDR4-3200 DIMM CL16-18-18-38 Dual Kit
* 1000GB Toshiba P300 High-Performance HDWD110UZSVA 7.200U/min 64MB 3.5" (8.9cm) SATA 6Gb/s
* 512GB Patriot P300 M.2 2280 PCIe 3.0 x4 3D-NAND TLC (P300P512GM28)

After turning the PC on for the first time we noticed that only the hard drive started to work. No fan, not the CPU fan nor any of the case fans (which are all connected properly as far as I can tell) start to turn and we got no output from the HDMI port.
The front panel is powered though (power is connected via a SATA cable that is connected to the same cable that powers the hard drive). What puzzles me is that the on/off-button on the front panel that is connected to mainboard works.

I'm out of ideas now.
Any help is very welcome!

Well that's a pickle.

You won't like what I'm about to say, but you gotta pull it all apart and rehook everything up. This time though, minimize. Leave the extra hard drive disconnected.

The front panel is getting power, the hard drive is on the same wire and sounds. Hmm. Kind of weird that works but none of the other fans turn.

Def pull everything out and try again. Don't even bother with the video card or video or at all, first step is to get power to everything. Once all the fans begin to work, shut down and put in the video card. Hopefully you'll get video output then.

Best of luck. Rough way to start your first build, but that's how this stuff goes, unfortunately!

Double check the motherboard and CPU power cables from the PSU are both fully clipped in, perhaps?

Phew, we reassambled the whole PC, - and it works!
Turned out it indeed helps to plug in *both* motherboard cables into the PSU ...

Thanks for your answers!

Yeah, that's sometimes a very simple thing to overlook.

Glad nothing is busted!