On our test/dev environment, I had a web application which I deleted and re-created. Using the Central Admin I re-attached the content database to the re-created web application. Problem is, visiting the site brings up a 404 error. If I visit a site admin page for the site (e.g. /_layouts/settings.aspx), I get a rather plain "An unexpected error has occurred." page. I fired up ULS and the admin page visit turns up a string of "Unknown SPRequest error occured. More information: 0x80070002". I googled that up, and while there were hits, none of 'em made the pain stop. Fwiw, this is SP2010, Apr 2012 upd.
I'm open to suggestions on what to try next, I've bing'd and googled this a hundred times today and am tired already!