downtime

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The period of time when something, such as a factory or a piece of machinery, is not in operation, especially as the result of a malfunction.
• noun A period of time when one is not working or engaged in a planned activity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The amount of time lost due to forces beyond one's control, as with a computer crash.
• noun A break or rest from work; leisure time.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a period of time when something (as a machine or factory) is not operating (especially as a result of malfunctions)