An item such as a database can be down or the network may experience a failure. There may be other circumstances where an item can become irresponsive. If this occurs and an item won't respond for a certain period the item gets disabled. Settings/Timeout page allows you to define the period in seconds before an item gets disabled

If an item becomes irresponsive as a result of a crash or a network failure it gets disabled after timeout. You may choose to have the item pinged at regular intervals and automatically reenabled once it becomes available again or leave it disabled