Problem with AirportExtrem!!!

Hi everyone, I have problem with my 2.4GHz Wi-Fi connection. I have the Airport Extreme for over two years and today out of nowhere it stopped working. I have setup a 5Ghz for my iMac and 2.4Ghz for other things like my iPad and iPhone, strangely the 5Ghz works just fine but the 2.4 Ghz stopped working and i can not connect my iPhone or my iPad to the internet. I can detect my Wi-Fi network but unable to establish any connection to the 2.4 ghz bandwidth. I opened airport utilities everything seems as it was before, How can i fix this? is there any thing that can cause this?
The internet is working just as it was, so it must be the Airport Extreme and not my iPhone/iPad or my Modem.
Thanks