If your login credentials are indeed correct, possible TCP/IP might be disabled somewhere in the configuration for your SQL DB.  Re-enabling might be the fix you need to get the DB ''online.''
As an administrator, try the following under your Microsoft SQL Server Configuration Manager (this is found under the Configuration Tools folder under the Microsoft SQL Server folder from the Start Menu:
 1. Once Configuration Manager is open,
    on the left side tree panel, click
    SQL Server Services.
 2. To the right should be the results
    pane.  Verify that Running appears
    next to the name of each service
    under the State column.
 3. If Stopped appears next to any,
    right-click the name of the service
    to Start the service.
 4. Next, back to the left panel, expand
    the SQL Server Network
    Configuration.
 5. Click on the Protocols for
    MSSQLServer (if you truly defaulted
    during install, this will be the
    name). It could be different if you
    changed it during installation.
 6. Once on Protocols for MSSQLServer,
    to the right you should see Protocol
    Names and Status.  Verify that
    Enabled is present for TCP/IP.
 7. Enable it if disabled by right
    clicking on TCP/IP name.
 8. Back to left panel and click on SQL
    Native Client Configuration and
    expand it.
 9. Select Client Protocols and validate
    that TCP/IP is Enabled.
 10. Enable it if disabled by right
     clicking on TCP/IP name.
 11. Last step is to then go back to the
     SQL Server Services in the left
     panel and in the resulitng right
     panel, find your SQL Server
     (MSSQLServer) or the exact name if
     you renamed it, and right click on
     it and click on Restart.  This
     provided that you did find that
     some of the TCP/IP settings were
     disabled.