With the release of Windows 10, quite a few developers will be reinstalling Windows. One of the gotchas to be aware of is that in Sql Server 2014 Express, they have changed the connection string to access LocalDB.
Rather than "Data Source=(LocalDB)\v11.0;...." it is now "Data Source=(LocalDB)\mssqllocaldb;..."
For more information:
http://www.michael-whelan.net/sql-server-localdb-2014-connection-string/