Why do my gallery photos only show have of the picture?

[AC Question]

Yesterday, my photos were all there and today my photos only show half of a picture! The rest is just grey or are lots of colored little squares or just simply lines
And that just happened to the ones I have in my SD Card. I need help! I can't loose all my pictures!! They are memories! :'(
How can this be fixed? WHAT can I do to recover them all! BTW my phone is a Samsung Galaxy Note 4.
I greatly appreciate any help whatsoever!!