Please check connection string first

<connectionStrings>
  <remove name="LocalSqlServer"/>
  <add name="LocalSqlServer" connectionString="Data Source=.\SQLExpress;Integrated Security=True;User Instance=True;AttachDBFilename=|DataDirectory|aspnetdb.mdf"/>

there might be other reason

http://support.microsoft.com/default.aspx?scid=kb;en-us;896613

http://support.microsoft.com/kb/2002980
http://www.aspdotnetfaq.com/Faq/fix-error-Failed-to-generate-a-user-instance-of-SQL-Server-due-to-a-failure-in-starting-the-process-for-the-user-instance.aspx