What information should I register if I discover that my site has been attacked?

Question

In the code below, if I get into the operator if, I can say with confidence that my site is under attack. What information is a good idea for registration?
Any recommendations on actions that can be taken to minimize the damage at this stage?

protected void btn_Search_Click(object sender, EventArgs e)
{
   if(tb_SearchBox.Text.Length > tb_SearchBox.MaxLength)
   {
      //What should i log?
      //What actions should I take?
   }
   //Otherwise search
}

Answer 1

For action, I simply redirect them to the error page.

, @The Rook . , ELMAH. ELMAH HTTP-, IP- .

Answer 2

ip log . , ip

Answer 3

-, Mod_Security , HTTP- IP-.