Are you sure they are gone and not actually just on the phone? Try restarting the phone (turning it off and back on) and then check the phone and the SD card for the images. Do not check by using the gallery, but either using the file manager app on the phone (if it doesn't have one you can get one from the google play store) or by connecting the phone to a computer via USB cord and search the phone and SD card from the computer. Personally I recommend the connected to a computer because it is much faster and better searching abilities.
Now, please note, if you had the phone set to sync to say a google account, then the images could also be there.