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As you can see in the posts on this thread, every instance this problem uccurred, it was a server issue, not a XOOPS issue.
Try to connect to the database with another application, and make absolutely sure that the information in the mainfile.php in the XOOPS site's root folder is coorect (database name, user, password, etc.) Mind the formatting (case sensitive) issues here.
Again, if this were a XOOPS related issue, we would have had many reports that were not due to bad hosting and server management.
Herko