Sometimes, when low space is available, the card read in that way, most raw image or small video are more in size these days…So, if it detects there is not enough space or card id full for one raw image---it would read that way, even though it is not actually “full”.
Well, I think your card got corrupted at the time when you were started to record video. At such situation, you can proceed further to format your card. You can either format the card on your camera or you can format it on your PC. To format card on PC, here find out the steps: How to Format a Memory Card