All Fish Died During Holidays

Ah Fruitbat, I'm so sorry for your loss.
It was nerve wracking to go away last week and leave them and having lost my first six fish I do feel for you.

My ammonia was showing when I got back and Sue did ask if we'd had a power cut but apparently not.
How long were you away for. I was away for 5 days with nobody caring for them.
If my power goes off, everything comes back on again when the power is returned. The only thing that does are the clocks which are always way off.

I'm so sorry to hear this.

There are a couple of things that would go wrong. The heater would have been off, and depending how cold the tank got, the fish could have become stressed. Though that would apply more in winter than summer.
If the filter went off the ammonia made by the fish would no longer pass through the media, and although the bacteria on the other surfaces would not be affected, there wouldn't be enough to remove all the ammonia, which would build up and potentially kill the fish if left like this for a few days. Once the first fish died, that would push the ammonia level up even faster.
If there was no filtration for several days, the bacteria inside the filter could have suffocated. I know they go dormant if deprived of food, and only die after several weeks without food, but i don't know how quickly they die when deprived of oxygen.

Does your filter always switch back on after being turned off? I once had a filter that wouldn't start after a water change, I had to manually start the impeller. If this happened to yours, even if the power had only been off for a few minutes, there would have been no filtration afterwards.

thanks for your remarks about my fish, you do get attached to the little buggers !!!  power had been off but don't know how long for but when my daughter came up they were all dead but everything was working she said, once power was restored the pump and filter etc come back on automatically ( just turned off for a few mins and everything turned back on ok).. it was only 3 days in to my holiday that it happened so max 2 days without anyone being there ... arranged  someone come up 4 times a week .. told my daughter to just put a little food in twice a week to keep the cycle going and when i returned reading was ...  ammonia .5 nitrites over 5 didn't test nitrates .. though through this my 2 amman shrimp survived !!  strange  , i will never know exactly why they died but as I'm an electrician will have to work out a emergency back up

Do you have anywhere you could temporarily house the amanos? The safest way to start again would be to add 3ppm ammonia and test next day to check the health of the bacteria colonies. But the amanos won't appreciate that much ammonia.

It depends on what caused the fish deaths. If something happened to kill the bacteria, allowing ammonia and/or nitrite to get high enough to kill the fish, you need to build the colonies back up again. You could go slowly as for a fish-in cycle; there should be enough bacteria left for a few fish, then once you know the levels are staying at zero you could then get a few more. I would be concerned if you went out and bought a tankful of fish all at once that there may not be enough bacteria for them all.

The food will have helped. It will keep the bacteria in there fed as it decomposes to ammonia, and if any had died off, it will encourage the ones left to multiply.