I can think of a number of things to try. No guarantee of success.
Card is full - Download images to computer. Then format card in camera and start again.
Faulty card. - Try a new card. Format card in camera
Corrupted images. - Download any files to computer. You may be able to recover them. Then format card in camera and start again.
Possibility the camera is not recognising images shot using another camera. Format card in camera and start again
Check firmware is up to date. Instructions should be in your manual or there are plenty of references on this site.
It is always good practice to format the card in camera immediately after downloading images to your computer.
Good luck
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