Just posting in case this helps anyone with similar problems...

I recently had a power outage which lasted around 1 hour. I've had a number of these since owning my TAPO kit so not unusual. 
Once the power was back I noticed none of my C420 cameras were working. All looked fine on the app until I tried to access them, this failed with 'tap to refresh' messages.

I looked at the H200 Hub that they connect to. Green LED as normal. In the app the microSD card was greyed out so inaccessible. I know from previous firmware upgrades I need to be patient with this (256GB card) - Problems with H200 update.

After a few hours the cameras were still unavailable and the hub showed the microSD card as greyed out. I tried a hub off/on at the power as well as reboot via the app over the next 6 hours or so. Still no luck. The hub was working for my movement sensors so it seemed to be just the SD card that was causing the camera problems. 

Left things overnight, but still no luck in the morning. I removed the SD card and reinserted a few times as per the troubleshooting guides - still showed as greyed out. 
I found that the cameras were accessible when the SD card was out of the hub!
I could read the SD card fine in a PC. As I couldn't format the card via the app I decided to use the PC. Used exFAT with the 'Quick' option unchecked so took a few hours. 

I was relieved that when I put this back in the H200 hub it was now available and the app offered to initialise it. After that all my cameras returned to normal.