label is not disappearing(small problem)

hi friends i have a login page in my website.login page has two required field validators,label(invisible) and a button.
when ever user wont enter details into textboxes.it will give message please enter username and password by required field validators. if user enter wrong username and password a invalid message(label) will appear. now if user enter nothing in textboxes and click button. the invalid message(label) is still on my page.my requirement is it should disappear.here is my code. Could anyone help me please.
 
page_load(object sender, evenargs e)
{
if(!IsPostBack)
{
lblError.Visible = false;
}
}
btnLoin_click(object sender, eventargs e)
{
con = new oracleconnection("connection string");
con.open();
string query = "select * from individual where user_id ='"+txtUser.Text+"'" and pass ...