The most common cause for the install wizard to appear on a previously working site is that DotNetNuke can no longer connect to the MS SQL Server database. There are many possible reasons for not connecting - here are a few common ones:

SQL Server service is not running.
Database userid and pwd  (for SQL Server Authentication) have changed either on the server or are not correctly set up in web.config. Most commonly when the database user login was created, the user's password was incorrectly set to expire after a certain perior of time.
Network connection error causing SQL Server to be unreachable.
Database performance issue resulting in request timeout - usually indicates that transaction log or DNN EventLog, SiteLog, ScheduleHIstory table have grown excessively large.

If you are not able to track down the cause of the problem and fix it, please indicate the following:

1. Is the DotNetNuke site and MS SQL Server installed locally or on a hosting provider?
2. What version of DotNetNuke is installed?
3. What version of MS SQL Server is installed?
4. Do the DotNetNuke web.config connection strings reference a user instance (AttachDbFilename attribute), integrated Windows Authentication (no Username or Password attributes) or SQL Authentication (Username and Password attributes)?
5. Do you have SQL Server Management Studio (or other database management tool) and if so can you connect to the database using the same connection string information as in web.config?

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner