Cold tank

[Dearth]

Not long after I left for work this morning I lost the power to my house for about 15 mins everything came back on except my heaters the backup somehow got unplugged and my main fried. I came home tonight to a tank that was 18c. So far everything looks alive but what are some concerns I should be watching for as I reheat the tank I currently have the water up to 24c?

I did a 20% water change and have my furnace going now as we are expected to be below 0 tonight.