It sounds like the OS may have been installed with RAID enabled and after switching to AHCI it would not boot to Windows. It may be that you will have to reinstall Windows but with the option to keep your files and settings. That can be done using the media creation tool found at MS here (Windows 10 version) or here (Windows 11 version). And don't ask me my opinion about BIOS updates. There is a reason that the major MB manufacturers warn to not update the BIOS unless it cures a specific problem that the computer may be having.
I have a XPS 8930  and after a BIOS update, although the computer still recognized my SSD main drive, it would no longer boot. After exhausting all options I still had no luck getting it running. I’ve had issues with updates before where the Boot order had changed but this time that wasn’t the issue. I began to suspect that the computer was attempting to boot from the HDD instead of the SSD but I saw no way to choose one drive over the other. So I simply unplugged the HDD and lo and behold it worked. By the way, both drives are OEM so I would not have expected an issue. Anyway, is there a setting to fix this in the BIOS without unplugging the HDD if this happens in the future?
Bios update problem