I had a lot of windows and tabs open in the Safari web browser on my MacBook Pro laptop running OS X 10.10.5 (Yosemite) as well as many other apps open. I've found when I leave many browser tabs open for a prolonged period that eventually they consume almost all of the system's memory. The Activity Monitor application was showing about 15 GB of the system's 16 GB of memory as in use. I closed a couple of tabs, but then opened another one. Then I got the "spinning beachball" and could do nothing further with Safari, so I clicked on the Apple icon at the top, left-hand corner of the screen and chose Force Quit and forced Safari to quit. That reduced the "memory used" value from about 15 GB down to about 5 GB, but I saw that many other applications were listed as "paused" in the Force Quit Applications window and I could no longer use those applications - I just got the multi-colored, spinning beach ball when I clicked on them or tried to access them by cycling through open applications with Command-Tab.
The Activity Monitor window showed those applications as "Not Responding".
The Terminal application is not shown as "Not Responding"
in the Activity Monitor window, though it is shown as "paused" in
Force Quit Applications because I unpaused it prior to taking the
Activity Monitor screenshot. To unpause applications, I needed
to access a Terminal window to issue "kill -CONT" commands. You can use
kill commands to stop errant processes,
but you can also use kill -CONT pid commands where
pid is the
process identifier (PID) of the process you wish to "unpause" to cause
a process to resume or "continue" its operation - see
Suspending and resuming a process on OS X.
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