Stuck in bios after changing from CSM to UEFI

With the BIOS in the modern more advanced and nowadays preferred UEFI mode, it requires the boot drive to have a so-called GPT. But as your BIOS previously was in the older CSM mode (which was only necessary for an old OS like Win7, or when you re-used a Windows installation from an older system), it will have prepared the boot drive to have a so-called MBR. So you have to do a quick MBR to GPT conversion for the Windows boot drive, then the BIOS in UEFI mode can boot from it again. It takes just a couple minutes to get this going again.

Maybe this is also a good opportunity to update the BIOS to the latest version, because that will (for most halfway newer boards) automatically enable UEFI mode, firmware TPM and Secure Boot, so you are ready for Win11. You will have to do the MBR to GPT conversion either way. Secure Boot is an UEFI function, and for UEFI to boot the OS from the drive, it has to have a GPT.