How to access form controls from class

Well, it's a bad practice that what you're trying to achieve.
Instead, pass the control values as a parameters to the function of your class, which would return something.
For example,

Login.cs

public string LogIn(string user, string password)
{
    string STATUS = string.empty;
    
    string query = "select * from User where UserName = '" + user + "', AND Password = '" + password + "'";
    SqlCommand cmd = new SqlCommand(query, con);
    
    int n = cmd.ExecuteNonQuery();
    
    STATUS = (n > 0 ? "Success !" : "Error");
    
    return STATUS;
}

WebLogin.aspx.cs

{
    Login l = new Login();
    string status = l.LogIn(txtUserName.Text, txtPassword.Text);
    Response.Write(stauts);
}

It is just the basic idea, how you can go about this. Hope this would help you.

-KR

Create Read/Write property in the login class as

public class Login
{
    public string UserName { get; set; }

    public void SomeMethod()
    {
        // read the username
        string username = this.UserName;
    }
}

and you can access the same(property) in the ASPX.cs by creating an instance to the Login class.

public void button_click( object sender, EventArgs e)
   {
       Login login = new Login();
       login.UserName = "karthik";
       login.SomeMethod();
   }