Try restarting the phone.
If that doesn't work, try connecting the phone to a computer, via USB cord, and then search both the SD card and the phone's "Internal" storage for the images. If they don't show on either, they could have accidentally be deleted (it happens more often than you might think). If that is the case, you can try the following to 'try' to recover them.
While there is no guaranty that you can get them back. Deleted does mean deleted. There are a few apps/programs out there that 'may' be able to help you recover the data.
A few of the programs you could try would be... DiskDigger, MobiSaver, Android-Recovery and Tenorshare. They are all data recovery apps/programs. There are many others out there, so check around...