Page loading on login failed error msg.

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C#

HTML

I have created a web application. Whenever any user tries to login into that application using wrong credentials, it should show alert" Please enter correct Userid and Password". But the problem is that, the page keeps on loading in the backend until the user does not click the "ok" button of the message. I want to avoid page loading after error message has been displayed. plz help.

What I have tried:

Below are my code:

HTML:
                                <asp:Button ID="btnLogin" runat="server" Text="Login" class="button" OnClientClick="return validateTextPw()" OnClick="btnLogin_Click" formmethod="post" />


C#:

        protected void btnLogin_Click(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["MmrtgDBConnString"].ConnectionString);
            con.Open();
            SqlCommand cmd = new SqlCommand("select * from MSTR_USER where UserId =@username and UserPassword=@password", con);
            cmd.Parameters.AddWithValue("@username", txtUserId.Value);
            cmd.Parameters.AddWithValue("@password", txtPsd.Value);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);
            if (dt.Rows.Count > 0)
            {
                Response.Redirect("Login.aspx");
            }
            else
            {
               ClientScript.RegisterStartupScript(Page.GetType(), "validation", "<script language='javascript'>alert('Invalid Username and Password')</script>");
            }
        }