The live session user has no password - it is named according to the operating system name, in lowercase ("ubuntu" or "kubuntu" or whatever) and their password is set to the empty string (nothing, or blank or whatever you want to call it).
If you ever log out by mistake, you can log back in by choosing the user icon from the graphical login screen and hitting the login button (or pressing ENTER on the empty password field), but if you got the lock screen triggered(*) then you can't unlock it because the lock screen password dialog will not accept an empty password.
Here's what to do instead (be careful - requires scary textual virtual terminals, and typing ):
- Press CTRL+ALT+F3 (or with F4 or higher - try until you find an open text vt - it takes a couple of seconds for the system to open the vt, so don't give up). It should say, at the top "Ubuntu VERSION.NUMBER" and then something, and at the second line "NAME login".
- Type the user name for the operating system live session user - it should be the name to the left of the word "login" but if it doesn't work, guess something else - it is usually something obvious.
- A "Password:" prompt should appear - just press ENTER and you should be in (unless you messed up the user name, then try again).
- At the command prompt that opens, type
sudo loginctl unlock-sessions and press ENTER. If you did it correctly, it will say nothing and open a new command prompt.
- Type
exit and hit ENTER. It should close the text terminal and go back to the empty screen with the login prompt.
- Press CTRL+ALT+F1 (or sometimes with F2) to go back to the main graphical session, which should now be open.
*) Given the situation with the lock screen, I think it's dumb that the live session user even has a default screen lock timeout - which, BTW, a default OEM installation doesn't. They should build live images without a lock timeout. If you ever plan on running something long using a live session (like, checking a large drive for bad sectors), make sure to inhibit the screen lock - in Kubuntu it's as easy as clicking the battery widget in the system tray and checking the "manually block sleep and screen locking" checkbox in the menu that opens.
