When the Terminal Fails
Ubuntu is a robust operating system, but occasionally, a full-screen application—particularly a demanding game running via Proton or Wine—will completely freeze. When the graphics server locks up, you may find that you cannot switch to a TTY console (using Ctrl+Alt+F3) or open a standard terminal to kill the offending process. Instead of reaching for the physical power button to perform a hard reset, you can use the kernel’s built-in emergency communication system: the Magic SysRq key.
Using the SysRq Emergency Quit
The Magic SysRq key allows you to send low-level commands directly to the Linux kernel, bypassing the frozen graphical interface. To force the kernel to terminate all processes on the current virtual terminal (which will kill your frozen game and return you to the login screen), press and hold the Alt and SysRq (often the Print Screen) keys simultaneously.
While holding them, tap the K key. This specific command (SAK, or Secure Access Key) forcefully kills all programs on the active console. Within a few seconds, the display server should restart, and you will be presented with the standard Ubuntu login screen, with your file system completely safe and intact.