1. Not operating. "The up escalator is down" is considered a humorous thing to say, and "The elevator is down" always means "The elevator isn't working" and never refers to what floor the elevator is on. With respect to computers, this term has passed into the mainstream; the extension to other kinds of machine is still hackish.
2. "go down" To stop functioning; usually said of the system. The message from the console that every hacker hates to hear from the operator is "System going down in 5 minutes".
3. "take down", "bring down" To deactivate purposely, usually for repair work or PM. "I'm taking the system down to work on that bug in the tape drive." Occasionally one hears the word "down" by itself used as a verb in this sense.