Switch to a terminal (Ctrl+Alt+F2) and do AltGr+PrtScr+H - if you've got Magic SysReq enabled that will print the help information (a list of commands with their (k)eycodes) to the terminal. If it doesn't print anything then you need to RTFM to find out how to enable it. AFAIR I had to do this shortly after I switched to Kubuntu as it was disabled by default - I often had to use Ctrl+Alt+Backspace or AltGr+SysRq+K to kill X as there's a hardware bug on my Nvidia graphics card.

My 8yo has a mnemonic for it something about Elephants and Umbrellas but I've always done reissub (now with extra sync'ing power). For a time the process for him was login, Alt+F2, konsole, Ctrl+R, mine, Enter, AltGr+SysRq+R, E, I, S, U, B; then repeat the first part and you're finally ready to play Minecraft!