I modified sys-usb not to start on boot, and now I can't log in

[ChrisA]

I believe the important advice is the second part of this post:

Locked myself out after creating Sys-USB

Simply add kernel parameter, on grub menu press e to edit, then add qubes.skip_autostart in line that has rhgb quiet, in the same line if there’s rd.qubes.hide_all_usb delete that, then f10 to boot with this parameter.

in the thread from @LEMON101 : removing rd.qubes.hide_all_usb from the GRUB menu …