@RichHaynes 

This light change isn't a normal LED state. Since you've confirmed that the smart plug is no longer connected to the WiFi network, i'm afraid the only option left now is a factory reset. 

If the LED still doesn’t light up after the reset, or if it doesn’t change normally during the reset process (blinking green), this is already an indication that this device needs to be replaced. To proceed warranty, please contact the retailer or local TP-Link support. 

However, this issue might just be a malfunction of the product's LED indicator, and the device itself could still be operational. If you'd like to give it a try, add a new smart plug in the Tapo app, see if the configuration goes through.

Note: Depending on the version, the smart plug can be set up via WiFi or Bluetooth. If it uses Bluetooth, the device won’t have a default WiFi connection. You can try both methods to see if either one works.