Find the name of the Volume Group (henceforth “somevg”) that contains the root Logical Volume:
sudo lvs
(If it’s not showing up, try running sudo lvmdiskscan and sudo pvscan then try again.)
If you run ls /dev/mapper/ you may see that the Logical Volumes is not showing up. If it’s not there, you need to run the following command to make the kernel aware of the logical volumes:
sudo vgchange --available y <somevg>