Hey there, let's break down your problem and work through possible fixes together:

First, let's unpack that error message:
The [FAILED] Failed unmounting /cdrom line means your system couldn't properly disconnect the /cdrom mount point during reboot. This almost always happens because the installation media (like the USB drive or DVD you used to set up the OS) is still connected and being accessed, or there was a small glitch in how the mount was configured.

As for your keyboard and mouse not working:
Since you landed at the command prompt instead of the normal desktop, it's likely the boot process got stuck halfway because of the /cdrom unmount failure. This means the system didn't finish loading the drivers or services needed to recognize your input devices.

Here are some actionable steps to try:

• Start with a clean reboot: Hold down your computer's power button until it shuts off, then unplug any installation media (USB/DVD) that's still plugged in. Boot up again—this might let the system skip the problematic /cdrom step and load normally.
• Check hardware connections: If you're using wired peripherals, try plugging them into different USB ports. For laptops, double-check there's no physical switch disabling your keyboard/mouse. You can also enter your BIOS/UEFI during boot to see if the devices work there—if they do, the issue is with the OS, not the hardware itself.
• If you can get command line access later: If the keyboard starts responding at the prompt, try these commands:
  1. Check what's mounted at /cdrom: mount | grep /cdrom
  2. Manually unmount it: sudo umount /cdrom
  3. Try launching the graphical interface: startx or sudo systemctl start graphical.target
• Verify your installation media: If the problem sticks around, your install media might be corrupted. You can use the installer's built-in integrity check option, or create a fresh installation drive, then try reinstalling (make sure to back up any important data first if you have it).

When I clicked reboot now, it took me to the command prompt screen and there one of line was
[FAILED] Failed unmounting /cdrom
Now I can't even use my keyboard and mouse.

备注：内容来源于stack exchange，提问作者imran khan