If a website that you manage analytics for is down, you should not be the last to know about it.
For one thing, you should make sure that the situation is being addressed, even if this is not in any way your direct responsibility. For another, you need to record the exact time and date of the occurrence and correlate it with any resulting anomalies in your Google Analytics data.
You can take advantage of many free and very low-cost monitoring services such as SiteUptime. In addition to real-time alerts, look for a daily and monthly reporting option when choosing a service.
Although not part of Web analytics per se, reliable and timely website monitoring should be included as an important item on your Google Analytics implementation checklist.