A couple of weeks ago, we got a call at 4pm. Our potential client had heard via e-mail at 3:15pm that their site was getting shut down by Monday and they should make alternative arrangements. It's a proprietary CMS (Content Management System) - no alternatives. Luckily, they had control of their own domain so we could get DNS moved to us. This was Friday.
Saturday morning we went to see what we could move. We hoped to grab a fixed copy of their site at the very least. Their site was already gone. It appears their Provider had gone under.
The Internet Archive (the Wayback Machine - good fun when you have some spare time) had never been able to index the site (proprietary CMS again) and couldn't help us. Querying Google's cache got us about 80% of the data, but not the functionality. We put together a fixed copy of their site, put up "under maintenance" for functional sections, and then started talking to our new client to see how they wanted to go forward now that we had survived the weekend.
So what lessons do we draw from this? Be careful about using a proprietary CMS.
Community supported CMSs provide good continuity, protection from most of the above issues, and the ability to change between providers if you need to.
We like to do other things on the weekend. I am sure you do too. You have been warned.