How can the homepage disappear off of one of the sites in a site collection?
/_layouts/viewlsts.aspx was still available, so I clicked "Site Actions", "Edit in SharePoint Designer", did the same in a working similar site, and copied "default.aspx" from the "All Files" section of the working site to the same of the broken one. Rightclick, "Set as homepage". Site restored to working order.
"Site Actions", "Edit Page", correct text and web parts. I get an unexpected error updating the "Contact Details" web part, but that's another question.