The most likely cause is a failing SD card or a card/reader write issue. If images were visible in-camera and later disappeared, that suggests the card’s filesystem or storage may have become corrupted. Another possibility is removing the card from a computer without properly ejecting/unmounting it first, which can leave writes incomplete and corrupt the card. A failing card reader can also contribute.

What to do:

• Replace the SD card. They’re inexpensive compared with losing photos, so don’t keep trusting a suspect card.
• Always eject/unmount the card properly before removing it from a computer.
• Consider the card temporary storage only; copy photos off and back them up regularly.
• If the problem happens again with a new card, investigate the card reader or camera.

In short: yes, getting a new SD card is the safest next step, and careful ejection plus regular backups are the best prevention.