Add event onselectedindexchanged on your combobox.
then in code behind i.e in .vb file
string partno = yourcomboboxname.SelectedItem.Value;
then use this partno to fetch whole row from db.
then add those value to your required text box.
if partno will be unquie then you get single row from database.
SqlDataReader reader = cmd.ExecuteReader("Select from tablename where partno = " +partno;
TextBox1.Text = reader["CoulmnName"];
Use same code in vb WITHOUT ";"
Hope this will solve your problem.....