Hi try this
in page load
if(!IsPostBack)
{
string sql="select sum(NetAmount) from table_name";
sqlcommand cmd=new sqlcommnad(sql,connectionstring);
cmd.connection=connectionstring;
con.open();
sqldatareader dr=cmd.executereader();
if(dr.Read())
{
textbox1.text=dr[0].tostring();
}
dr.close();
conc.close();
}
it will work for sure..try this..
if it is ur answer accept solution..
happy coding..