Why are some of these photos suddenly just gray with a cloud check mark icon?

[Android Central Question]

It's just in this one album too. I was trying to free up space on my phone so I was getting ready to upload them onto OneDrive just in case they hadn't been, before deleting them on my phone, and I noticed that it didn't upload 94 photos (onedrive said 266, there's 360 in the album on my phone) and that's when I noticed that quite a few of them had been turned into this gray box. I can neither download the affected photos nor upload them to OneDrive. How did this happen? Can I get them back?

 

[B. Diddy]

Welcome to Android Central! Which phone? Are they saved to Internal Storage or an SD card?