Black screen after graphics card change

"on this motherboard, the GPU is silent and therefore disabled in the BIOS (I suppose)"

I assume you tried connecting the monitor to the motherboard. If that's the case, it's normal that you're not getting any signal. With a dedicated GPU (like your Asus RTX3060 Ti), the BIOS disables the IGP (integrated graphics processor) to save resources. Since the video outputs are no longer connected to a graphics processor, they become inoperative. The monitor must be connected to the dedicated GPU.

Finally, if everything is properly connected, try a Clear CMOS or remove the CMOS battery for 30 minutes, for example.

"I thought that the cable via the hard drives was poorly connected (like master after slave) but I deduced that this system which existed in the past must be outdated today (no longer exists). Not having access to anything at all, neither the OS nor the BIOS, I am at a loss. Do you have any idea how to solve my problem?"

A poorly connected drive does not prevent the computer from displaying. Moreover, unless your PC is from the dinosaur age of PCs, the master/slave configuration no longer exists in hard drives. In SATA drives, each drive has its own controller. I assume you are connecting your monitor to the RTX 3060 Ti and not to the motherboard. In that case, check that your RTX 3060 Ti is properly seated in the PCI-Express 16X slot on your motherboard and that the 8-pin PCIe power cable from the power supply is properly connected to the GPU's socket.

thank you for your response epango ;-)

indeed the GPU is well seated on the motherboard. for the GPU power connection, it's a 6+2 and I also thought about reconnecting the 4 pin power connector to the motherboard. as for the HDD, I had a feeling that since 1985 (when I was 15 years old with the Amstrad CPC 464) PCs had evolved significantly :-D, thank you for your clarification. on this motherboard the GPU is silent and therefore disabled in the BIOS (I suppose). it's worth noting that every time I work on the motherboard, I discharge any electrical current to avoid any shock.

Tonight I will try some manipulations as you suggested just to reassure myself, but for the past 3 days I've been struggling badly with the beast ;-)

thanks again for your advice and assistance...

Looking forward to hearing from you,

Laurent

I'm trying the clear CMOS...

Well, here's the latest news; after yet another check of my connections, the RAM was not properly seated. And now the PC is back up. So I could activate the GPU on the motherboard just in case...

Thanks everyone...