modern-web-design-and-development

then your server may have run out of workable memory, especially if the
swap space (a bit of the hard drive that the server uses for extra memory) is
full, too.

If your server has run out of memory, then the top command will identify
which bit of software is being greedy.

admin@server$ top

Every few seconds, this gives a snapshot of which bits of software are
running, which user started them and how much of your memory and CPU
each is using. Unfortunately, this will run very slowly if memory is low. You
can press “Q” or Control + C to exit the command.