Hi bloggs!
If you have not yet selected the AC ESS feed pushbutton to ALTN, then you will have lost both AC and DC ESS busses. Therefore you will have lost the supply to the Standby Horizon. It will still continue to work for a while though until it loses RPM.
The AC ESS feed pb transfers AC ESS bus to AC BUS 2. The DC ESS bus is powered from the AC ESS via the ESS TR!
If you have already selected the AC ESS feed and you still have the red flag in the AI then I'm afraid I'm stuck too!