ExecuteReader requires an open and available Connection. The connection's current state is closed.

Make sure that cn (your connection) is open.
Before dtr1 = comm1.ExecuteReader(); put the line cn.Open() and then try again.

you can change this way.. when you read database first you want to open the connection using [SqlConnectionObject].Open();, In your code you want to add cn.Open();. when you finish data accessing you need to close your connection by [SqlConnectionObject].Close();.

and try to use try-catch-finally[^] for sql operation so you can handle your exception too..

public int foundmer(string mername)
   {
       try
       {
           cn.Open();
           SqlCommand comm1 = new SqlCommand();
           comm1.CommandType = CommandType.Text;
           comm1.CommandText = "select cse from tbl_user_CSE where cse='" + mername + "'";
           comm1.Connection = cn;
           SqlDataReader dtr1;
           dtr1 = comm1.ExecuteReader();
           if (dtr1.HasRows == true)
           {
               dtr1.Close();
               return -1;
           }
           else
           {
               dtr1.Close();
               return 1;
           }
       }
       catch (Exception ex)
       {
           //log your exception here
           //eg: label.Text=ex.Message;
       }
       finally
       { cn.Close(); }
   }

you forgot to Open the ConnectionState of your SqlConnection. Try adding cn.Open() before dtr1 = comm1.ExecuteReader();

Regards,
Eduard