error: ORA-03113: end-of-file on communication channel

Hi guys
I am getting error :ORA-03113: end-of-file on communication channel.
On page load i am showing counting in Label.
here is code
Dim site2 As String = "select count(*) from lettermaster where id in (select id from   ( SELECT id,office_code, is_important, RANK() OVER (ORDER BY id DESC)  as sal_rank  FROM lettermaster where is_important='1' and office_Code ='" & office98 & "' ) where sal_rank<=10)   "            Dim Count2 As Int32 = 0            conStr = "Provider=OraOLEDB.Oracle.1;Datasource=;User ID=;Password= "            Using con As New OleDbConnection(conStr)     &nb ...