Check to see that the specified instance of SQL Server is online. 2) If your SQL database files do not have the .mdf, .ndf, ...

1) Check to see that the specified instance of SQL Server is online. 
2) If your SQL database files do not have the .mdf, .ndf, and .ldf extensions for the primary data file, secondary data file, and log file, respectively, then DPM will not be able to attach the database to the specified SQL Instance.