What you didn’t mention was whether you were using a wireless mouse and keyboard or one that is using one of the USB ports.
If using wireless, you might have a wifi issue, either the wifi in the machine software is corrupt or there is a hardware issue, or the batteries are dead in the control devices.
If using wired control devices, you might try using a different set (keyboard and mouse) from a known running system, and try them on a different USB port.
Did you upgrade the system to Big Sur, or did you get the machine with Big Sur already installed?
If you shut the machine down by holding the power button down (provided you have no control of the screen to initiate a proper shutdown) and then do a restart, do you get the startup chimes, or some sort of coded beeping? If you get the chimes, the machine is happy with the ram.
If the machine starts with the chimes, does it go to a log in screen, or if you’ve selected to auto login, does it go to your desktop? Or do you get a question mark or some other odd looking icon?