DIAGNOSIS Alternating green/red flash is a specific error state. It means the hub is online (green) but has lost communication with the paired door sensor (red). The hub itself is fine, but it can't tell if the door is open or closed.
FIX This is a sensor re-pairing job. First, replace the sensor's CR2032 battery—even if it's not dead, this resets it. Then, press and hold the button on the back of the door sensor until its LED blinks rapidly. Within 30 seconds, press the button on the front of the myQ hub (under the light) once. The hub light should spin blue, then settle to solid green. The sensor LED should go solid. Check the app; door status should be back.
IF THAT DIDN'T WORK There might be wireless interference from something new in your garage, like LED lights or a new router. Try moving the myQ hub to a different outlet, a few feet away from its current spot, and run the re-pair process again from there.
HEADS UP If you have multiple garage doors with multiple sensors, the flashing red/green means it lost the *primary* door sensor. The hub button will only re-pair the sensor it's currently erroring about. Make sure you're pressing the button on the sensor for the correct door.