big sur installation failure now frozen in recovery mode

I have an iMac. The Big Sur update failed during install. Now it automatically boots to recovery mode. The keyboard will work to acknowledge that the machine is in recovery mode, (this is already highlighted upon boot, so just need to hit enter), but no further navigation is possible from keyboard or mouse. The various recovery action options appear on the screen, but I cannot get to them. The keyboard is plugged into the machine and didn’t work as previously described. The mouse will not work under any circumstances. The pointer remains stuck in the upper left corner.