When you performed the reset, you returned the Express back to its original new, out of box settings. So, you have to reconfigure it again to with settings for your network. You will not see a green light until you complete this process.
Temporarily, connect a spare Ethernet cable from one of the LAN <-> ports on the AirPort Extreme to the AirPort Express
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click Manual Setup
Click the Base Station tab below the row of icons and assign a name to the Express, a device password and adjust Time Zone settings
Click the Wireless tab and adjust settings to look like this:
Wireless Mode = Extend a wireless network
Wireless Network Name = Same name as the AirPort Extreme wireless network
Enter a check mark next to "Allow wireless clients"
Wireless Security = WPA/WPA2 Personal
Wireless Password = Same password as the AirPort Extreme wirelesss network
Confirm Password
Click Update at the lower right
As soon as you see the message that the settings have been stored successfully, close AirPort Utility and disconnect the Ethernet cable
Wait 40-45 seconds for a green light. Now you can move the Express back to the desired location and plug it back in