AirPort Express orange yellow light problem and not discoverable at all
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.