When we deal with huge amount of data, generally it may occur TIMEOUT problem... in those situations Take care of 3 things ( problem will be almost solved )
put the connection timeout property in Connection string string connstring=" Data Source=localhost;User ID=sa; pwd=secret; Initial catalog=nothwind;connect timeout=900;";
set the command timeout for the SqlCommand Object com.CommandTimeout=0; // this will makes Command to execute unlimited seconds
Now .... last but not the least... in the form unload function destroy the connection object conn.Dispose();