Unable to add data into database - sql-server-2008

Question

I created a 8 column table in SQL Server 2008. I entered data into 1st 2 column of the table and remaining columns I left allow nulls.
I am trying to add data to remaining 6 columns based on the data entered in 1st 2 columns, but I'm not able to add the data. Was leaving another 6 columns into "Allow nulls" caused this problem.
If yes, is there any solution for this?
Thanks.
string str = (#"Data Source=.\;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True;");
try
{
String sql = "(insert into usn (firstname, lastname, password, address, bloodgrp, contactnum, email) values (#st1, #st2, #st3, #st4, #st5, #st6, #st7) WHERE usn = '" + omd + "' )";
SqlConnection conn = new SqlConnection(str);
conn.Open();
SqlCommand cmd = new SqlCommand(sql, conn);
{
cmd.Parameters.AddWithValue("#st1", TextBox1.Text);
cmd.Parameters.AddWithValue("#st2", TextBox2.Text);
cmd.Parameters.AddWithValue("#st3", TextBox3.Text);
cmd.Parameters.AddWithValue("#st4", TextBox10.Text);
cmd.Parameters.AddWithValue("#st5", TextBox6.Text);
cmd.Parameters.AddWithValue("#st6", TextBox7.Text);
cmd.Parameters.AddWithValue("#st7", TextBox8.Text);
cmd.ExecuteNonQuery();
Response.Redirect("accountcreated.aspx");
conn.Close();
}

Answer 1

If you already have records in your table, you need to use UPDATE, not INSERT.
So, your query should be something like
UPDATE usn
SET firstname = #st1,
lastname = #st2,
password = #st3,
address = #st4,
bloodgrp = #st5,
contactnum = #st6,
email = #st7
WHERE usn = ...
You can't INSERT into a column, you use INSERT only to put data into a table (creating a record).
So, your sql String should look something like this.
String sql = "(UPDATE usn SET firstname = #st1, lastname = #st2, password = #st3, address = #st4, bloodgrp = #st5, contactnum = #st6, email = #st7 WHERE usn = '" + omd + "' )";