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.