@MootSeeker,

What LEDs are lit on the Range Extender? Are you able to still connect to the RE's wireless network?
 

If you can access the Range Extenders Web Interface or the settings through the web interface, navigate towards the settings where you fill out the credentials of your regular Wi-Fi and reconnect the RE to the Router.

If it seems to not work at all anymore, I would recommend performing a factory reset and attempting to configure the device from scratch. The easiest method is to set up the device with WPS - which just requires pressing the WPS button on your Router and Range Extender, and the RE should automatically connect. Otherwise, simply reconfiguring the network should erase any settings that were causing problems with your Range Extender's communication with the Router