ASP.NET issue facing exception issue

Question

I'm facing 404 error. It is throwing the exception error how tp resolve the exception error. I'm running Login page but it is taking to the error.aspx page in that its showing the exception what steps to be taken to solve the error. I tried in so many ways but couldn't get that if any of us know what is the exception then please let me know.

protected void Page_Load(object sender, EventArgs e) { 
  lblYear.Text = DateTime.Now.Year.ToString(); 
  Exception caughtException = (Exception)Application["TheException"]; 
  //
  string errorUrl = Application["ErrorUrl"].ToString(); 
  var errorUrlObject = Application["ErrorUrl"]; 
  string errorUrl = (errorUrlObject != null) ? errorUrlObject.ToString();
  string.Empty; 
  string UserId = Application["UserID"].ToString(); 
  // Here it is hitiing i dono how to solve the issue