There are four types of passwords that can prevent you from booting. Harddrive errors (/etc/fstab) also can trigger the Maintance Mode when trying to mount a drive.Root password. This is the system root account, but in LM does not exist, thus you use sudo to act as root, with your user password.
Perhaps you are asked this during a normal boot,and it says stuff about maintance mode and mounting harddrive. In that case use the password you used when creating the first account during inital install. Or sometimes you can 's' for skip. If skipping works, then make sure for fix /etc/fstab.
It looks like you have a BIOS password, in that case get your screw driver.
BIOS password. This usually requires looking for a jumper, or a clock battery to remove from the motherboard.
Hardware Harddrive Lock password. Involves buying a new harddrive because its physically locked from the motherboard. Unlockers exists, but you need special hardware.
GRUB Password. This is a password many redhat and server like distrobutions use to protect the first level of boot from being changed.