I am no sure what could have happened, but maybe you can retrieve them with Shadow Explorer (
ShadowExplorer.com - About) If you need operational details, watch this video tutorial (
Tutorial: Recover lost files with Shadow Explorer « How-To Geek Forums ). In addition I hope you still have the photos on the card in your camera. I never delete them from the SD card. With the low price of those cards these days, it is not worth deleting them.