Code:
on *:text:*:#: {
  if ($1 == !countdown) {
    if ($2 <= 5) {
      var %x = 1
      var %y = $2
      while (%x <= $2) {
        .timer 1  %y msg $chan %x
        if (%x == $2) { .timer 1 $calc(%x + 1) msg $chan GO!!!! }
        inc %x
        dec %y
      }
    }
    elseif ($2 > 5) {
      msg $chan 1,12Sorry 13,12 $nick , 8,12but max countdown number for me is 5 !
    }
  }
}

i tested this! and it works!!
Your Original Script Had Some Wrongs Closing Brackets.
Code:
on *:text:!countdown*:#: {
if ($2 = 5) {
}
msg $chan Countdown started !
timer 1 1 msg $chan 5
timer 1 2 msg $chan 4
timer 1 3 msg $chan 3
timer 1 4 msg $chan 2
timer 1 5 msg $chan 1
timer 1 6 msg $chan Go !
}
;;Basically the on text event ends here because you have close the: if ($2 = 5) { right below it's self. Not big deal .. i have made worse mistakes :D

The Little Difference Between What I wrote Is that you can have a countdown from 5 or lower
e.g:
Quote:

|17:05:41| OrFeAsGr › !countdown 5
|17:05:42|  Athens › 5
|17:05:43|  Athens › 4
|17:05:45|  Athens › 3
|17:05:45|  Athens › 2
|17:05:47|  Athens › 1
|17:05:48|  Athens › GO!!!!

or
Quote:

|17:05:57|  OrFeAsGr › !countdown 3
|17:05:59|  Athens › 3
|17:06:00|  Athens › 2
|17:06:01|  Athens › 1
|17:06:02|  Athens › GO!!!!

I Hope I Helped! Good Luck smile
-OrFeAsGr-