AirPort Express orange yellow light problem and not discoverable at all My AirPort Express is showing a permanent orange light. Every ~5 minutes it blinks green once and after this I get continuos yellow.

The AirPort device is connected via LAN to my Linksys WRT-160NL router that is configured with DHCP enabled - is supposed to give and IP to the AirPort.

The AirPort is undiscoverable via LAN or WiFi.

If I try the hard reset procedure, all I get is flashing yellow led after few seconds but no other effects.
Is it older than a year? AE power supplies eventually fail due to internal heat.  At least one failure mode - the one I recently observed - is just as you described it.  If I kept trying, I'd occasionally get a green light and Airport Utility would discover it long enough for me to set a configuration, but it would soon fail again and go back to a slow-blinking yellow light.  It would happen just often enough to keep me trying to revive it, but I finally let it go after sinking a couple of hours into it. At least in the US, if it is being used with a system that has AppleCare coverage, I believe the AE will be covered too. I hope that option is open to you.