how to display next record on button click

I think you suffer from a short term memory loss problem. This is a repost...
You have posted the same question with same code earlier and users have provided you with correct solutions here[^].....

But you posted the same question again. I can guarantee that the solutions are correct. Just check if the database has the data for the selected criteria and learn some basics of ado.net from here[^]
Debug the code line by line and check if every object is assigned its expected value? Check the exception if any..

Your code should be like this.

int recordindex =0;

string query = "Select * from ACCOUNTS where Account_number ='" + comboBox1.SelectedItem + "'";
//MessageBox.Show(query);
da1 = new SqlDataAdapter(query, cn);
dt1 = new DataTable();
da1.Fill(dt1);

    

public void Display_record(int i)
{
    
               comboBox1.Text = (dt1.Rows[i]["Account_number"].ToString());
               txtcustomername.Text = (dt1.Rows[i]["Name"].ToString());
               txtoccupation.Text = (dt1.Rows[i]["Occupation"].ToString());
               txtfathername.Text = (dt1.Rows[i]["Father_name"].ToString());
               txthusbandname.Text = (dt1.Rows[i]["Husband_name"].ToString());
               txtcustomercnic.Text = (dt1.Rows[i]["Customer_CNIC"].ToString());
               txtinitialdeposit.Text = (dt1.Rows[i]["Initial_deposit"].ToString());
               dateTimePicker1.Value = (Convert.ToDateTime(dt1.Rows[i]["Date"]));
               txtpostaladdress.Text = (dt1.Rows[i]["Postal_address"].ToString());
               txtemail.Text = (dt1.Rows[i]["Mail"].ToString());
               txtmobilenumber.Text = (dt1.Rows[i]["Mobile"].ToString());
}



private void button1_Click(object sender, EventArgs e)
{
              
              if (recordindex >= dt1.Rows.Count-1)
              {

                  //show end of record message.               
               }
              elseif (recordindex == 0)
              {
                 Display_record(recordindex);               
              }
              else
              {
                  recordindex++;
                  Display_record(recordindex);
               }

 }   

All The Best!!!!!

Here is a skeleton of an approach that I would use. http://www.functionx.com/aspnet_csharp/articles/parameterized.htm[^] is a place to start for paramterised queries.

//global variables
int RecordPosition = 0;
int MaxRecords = 0;
DataTable records = null;

public void GetData()
{
  string Query = "your select statement";
  //BUT MAKE SURE YOU USE PARAMETERISED QUERY if you have a where clause that
  //is dependant on your forms values

  da1 = new SqlDataAdapter(query, cn);
  records = new DataTable();
  da1.Fill(records);

  MaxRecords = records.count - 1;

  DisplayRecord(RecordPosition);
}

public void DisplayRecord(int Position)
{
  //Do your displaying HERE

  //increment the record pointer
  RecordPosition += 1;
}

public void ButtonClicked(object sender, EventArgs e)
{
   //Test that the RecordPosition isn't Greater than the MaxRecords
   //if it passes that test then
   DisplayRecord(RecordPosition);
}