I messed up my GRUB and BIOS

I was trying to boot to a USB with live Debian, which was not working (incriminating details on request). After running Grub Customizer, I rebooted but on the Grub page, the keyboard was unable to move down the list to boot to any other OS but the first. So I went into BIOS to see if I had done something. That’s when it got really bad.
I selected “Enable USB” in the Fast Boot section, and now I cannot get into BIOS at all.
I tried running the Ultimate Boot CD, which booted, but I cannot move up and down there either. Same keyboard silence.
When I let the machine boot, it opens Mint and both keyboard and mouse work just fine. Not so bad, but I need to boot into Windows too.
I tried plugging the keyboard into a different port, no difference.

So I created two problems. One is the keyboard does not work in Grub, and the other (probably more serious) is that I cannot get into any of the BIOS windows from a keyboard command.

So is it a wired usb keyboard, or wireless?
do a google of
‘bios without keyboard’
The links there seem helpful

Your fix might work if Windows was the first partition (it is the 4th on sda).
I could change grub to boot first into Windows but then I would not have Mint.
The non-keyboard fixes all would take me to UEFI, but what if I have Legacy/MBR boot? When I run efibootmgr, I get the following:
EFI variables are not supported on this system.
And I still have that issue of the keyboard not working until the OS has started.

What about if I take out the CMOS battery for a bit? That might fix the BIOS problem but not the silent keyboard issue.

You want to get into the BIOS, and while there alter that setting “Enable USB” back to what it originally was.
So yes try the CMOS battery trick.
If it works and you get into the BIOS menu, hopefully you will be able to use the KB or mouse to alter the setting. If not I dont know what to do?
Once the setting is altered, the KB should be OK and you should be able to choose items in the grub menu. I dont think there is anything wrong with grub, you have just lost KB control during the boot sequence.
During boot, the KB and mouse and screen are controlled by the BIOS. Linux takes over when it boots.

Fixed. The solution was to remove the CMOS battery.
When I put it back in, the computer went to BIOS right away, and FastBoot was not selected. THen I used the mouse to re-set the boot order, exited and the kb was working again on grub. I opened Windows and it reported detecting both kb and mouse.

Great. So removing the battery forces BIOS back to factory settings, because it cant remember your settings without power.
Good on you Cliff