System hang

In computing, a hang or freeze occurs when either a single computer program or the whole system ceases to respond to inputs. In the most commonly encountered scenario, a workstation with a graphical user interface, all windows belonging to the frozen program become static, and though the mouse cursor still moves on the screen, neither typing on the keyboard nor clicking the mouse produces any effect in the program's windows. The mouse cursor may also be stuck in a form indicating that it is waiting for some operation to complete, such as an hourglass or a spinning wait cursor.

Many modern operating systems provide the user with a means to terminate a hung program without rebooting or logging out. In more severe hangs affecting the whole system, no window belonging to any program will respond to keyboard or mouse input, and often the mouse cursor will freeze in place on the screen. Almost always, the only way to recover from a system freeze is to reboot the machine, usually by power cycling with an on/off or reset button.

In the Windows 7 and Windows Vista operating systems, hangs almost always precedes a Blue Screen of Death or are a sign to the user that one is on its way[citation needed]. On Windows systems, if clicked on, a hanging program will gain "Not Responding" in parentheses on its top bar.

A hang differs from a crash, in which a program exits abnormally or the operating system shuts down.
Hangs are not limited to client personal computers with a graphical user interface, as in the example above. Servers can hang as well. In those cases, the server ceases to respond to requests. These sorts of hangs are typically addressed by a solution far more complex than an on/off or reset button.

Pre-emptive multitasking operating systems, such as Microsoft Windows 2000/XP/Vista/7, Apple Computer's Mac OS X and Linux hang less often as the multi-tasking system is not affected by non-terminating loops and further does not require tasks to yield control to the operating system. If a task does hang, the scheduler may switch to another group of interdependent tasks so that all processes will not hang.