SD card is not a stable storage media, it's better to backup your data frequently. Here are two suggestions you can try:
· unplug your SD card from phone and plug into another card reader, then connect to a computer to see if there's any difference (backup your photos if you see them in computer)
- re-plug your SD in the phone, restart your phone and format your SD card on the phone. Try again to save photos on the SD to see if they are visible on Gallery app
· if you still can't see those files on your SD, try to use another 3rd party app like "Restore image" (type "file recovery" in Google Play to search) to recover your files