Today, I updated some wpmu plgins in one shot: videos, dashboard, subscribe by emails and post indexer. Everything seemed OK. I planned to "upgrade network" later when traffic was lower. Unfortunately when I came to do the upgrade network, I discovered that our front page had "disappeared". The content is still in the page file, but it can't be viewed or previewed. Checking further, I discovered the following message under plugins.
The plugin wdp-un.tmp/post-indexer.php has been deactivated due to an error: Plugin file does not exist.
All plugins have disappeared. So far as I can tell, the subsites seem OK.
Going to do a full site restore