Use your install disk to boot the "live" version of Ubuntu-mate.
open a terminal (Ctrl+Alt+T) and use the following commands,
sudo fdisk -l
(to list your partitions and identify where your root file system is, probably /dev/sda1 but not necessarily.)
sudo mount /dev/sda1 /media/linux
sudo chroot /media/linux
pwd (should return / )
(it will ask you to repeat it)
(look in /etc/password if you have forgotten your username!)
This has worked for me in the past, not sure if it still does but worth a try.