Unfortunately if you didn't back them up and they were erased off of the phone / SD card then you would be unable to retrieve them. In the future always ensure to backup your photos to Google + or Dropbox.
You might be able to recover them by using a program like DiskDigger. http://diskdigger.org/ Unmount the card (Settings/Storage), remove it, and insert it into your computer. Then run DiskDigger and see if you can recover the folder.
Posted via Android Central App.
Please excuse any typos or autocorrect derps.