The underlying provider failed on Open

When you get the following error message in Microsoft Visual Studio 2013 check the inner exception.

System.Data.Entity.Core.EntityException was unhandled
  HResult=-2146233087
  Message=The underlying provider failed on Open.
  Source=EntityFramework

The inner exception gives you detailed information on the real cause of the error. Most of the time the inner exception is the following:

InnerException: System.Data.SqlClient.SqlException
       HResult=-2146232060
       Message=Cannot open database “…” requested by the login. The login failed.
Login failed for user ‘…’.
       Source=.Net SqlClient Data Provider
       ErrorCode=-2146232060

Check the rights of the user. Most likely the specified user does not have access to the database.