So read the data from your DB:
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand cmd = new SqlCommand("SELECT Password FROM DeptTable WHERE Department = @DEPT", con))
{
cmd.Parameters.AddWithValue("@DEPT", myDropDownList.SelectedValue);
string deptPass = cmd.ExecuteScalar() as string;
if (deptPass == tbPassword.Text)
{
... all ok
}
}
}