Yeah. It’s not hardware, then.
I would try searching “black screen <bios version>” with any name and version number you can figure out about your bios, next.
If you can get it back to booting from install media, I would do a full reinstall.
There’s recovery layers (such as grub shell) that ought to kick in if this was just a display config issue, so I’m thinking corrupted install files is more likely.
Also, do a careful check through your various BIOS settings - search each one with “Debian 12 <setting name>”, to find out if they work with Debian 12, or need adjusted. Debian 12 supports most boot security features, that I have encountered, but I believe there’s still a couple out there that have to be turned off.
I suspect your next practical goal will be to get the (presumably failed) bootloader install replaced.
Edit: Tried to add a lot of specific thoughts as search term leads.
“The pirates code… Be more like… guidelines.”