Can't connect to SQL Server 2008R2 on Windows Server 2008R2

Solution 1:

Isn't that the default instance, for which you DO NOT GIVE A NAME IN THE CONNECTION STRING?

Solution 2:

Start->Programs->SQL Server->SQL Server Configuration Manager->SQL Server Browser->Properties->Start Mode = Automatic

Solution 3:

Use an alternative representation of the name of the local host address instead of "localhost". Possible alternative representations include the following:

• 127.0.0.1
• (local)
• "."
• The actual local host name

For example in your server try to change Server name field like so :

(local)\MSSQLSERVER or .\MSSQLSERVER

For predefined instance type you need to use only : localhost or 127.0.0.1

have you enabled REmote Connection to SQL Server ?

you could also try with this step-by-step guide : http://www.linglom.com/2009/03/28/enable-remote-connection-on-sql-server-2008-express/