Are your photos saved to internal memory or the SD card? (Camera / Settings icon / Storage (phone or memory card)
If it's memory card, was your phone plugged into a computer when you noticed this? The SD card unmounts when connected to a computer and any photos on the external SD card would not show up in the gallery at that time.
If it's memory card, you might also try removing and reinstalling the SD card. A poor connection might not allow the card to be read and reseating it may help.
If it's phone, have you recently performed a factory reset? A factory reset will erase all internal memory, including any photos saved in the phones memory. Note: A factory reset will not erase the external SD card.