Why have photos in my gallery turned grey?

After I got my iPhone 7, all of my photos had transferred from my old phone. I've been able to view all my photos ever since then.

However, last night I went to send a picture and noticed quite a lot of my images have done this. (See attached image). It's currently running the latest version of iOS 10.

It doesn't matter which album they're in, or what date, it appears to be a random selection of photos that have turned blank.

Is there any way to recover these photos. Also, my last back up was when I got the phone due to a very poor internet speed.

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