Why are my pictures not available?

[AC Question]

Hello, I have a HTC One M8 and lately every time I take a picture then view it in the gallery it is a small thumbnail when I open it. THen when i look at it again later all the thumbnails are grey tiles with a triangle and ! and when I try to open them, it says the media is not available. I have tried to view them on the computer, and they still do not work. The photos are stored on the sd card.

Is there any chance I can get these photos back?