See if the files are readable on another system and/or application as it could just be an incompatibility problem. Some have come across problems with some readers.
If you inadvertently powered off the AC without stopping a recording, the AC will not have properly finalized the video and so appear unreadable. The AC will fix this if it's powered back on with the same SD but will likely lose track of the file if the AC is powered on with no or different SD and so files showing corrupt could pile up.
The raw video data is still in the file so there are ways to repair the file with the needed headers if indeed it shows unusable because it's missing the headers.