@Jandj,
Smart Actions depend on their connection to the internet and cloud.
Have you made any recent changes to your network or security settings?
What is the status of the LED on the outlet
If you disconnect your phone from your Home Wi-Fi, are you able to control the plug? If not, it has become disconnected from the internet.
You may consider simply replugging the device, or performing a soft reset of the device, which would force the device to communicate with the cloud again but would save any settings, names, and automation you have set.