If you have the "new" AirPort Express with both a WAN and LAN port on the back, temporarily connect an Ethernet cable from your computer to the LAN <-> port on the AirPort Express to see if AirPort Utility can now "see" the Express.
You may need to click the Other AirPort Base Stations tab and select Ethernet to establish a connection.
Turn the wireless back on and Update to save the settings.
You can now remove the Ethernet cable and log back on to your network.
If no luck, you will need to perform a "hard reset" on the AirPort Express by holding in the reset button for 7-8 seconds, then release and allow a minute for the Express to restart to a slow, blinking amber light.
Now configure the Express again using the original settings that you used before.
Everything above assumes that you are using Lion or Mountain Lion on your Mac. If you are using Leopard or Snow Leopard, then the instructions will vary a bit.