This if a Fast Display, On Veiw menu>Diplay Performance>change to: Typical Diplay or High Quality Display
Could be a couple of reasons. First would be you've opened a .idml or .inx file and the links are missing. Those formats don't embed the image previews, so they need to be rebuilt and that can only happen if ID can find the links. Second woud be your Display Performance setting in the VIew menu is set to Fast. With Fast display you get only gray boxes to show locations and speed screen drawing. If it isn't one of those, check the prefs for the dipaly quality listed and make sure it hasn't been edited to show a proxy for the preview when it should show the image.
I had the the same issue after I upgraded from InDesign CC 2014 to InDesign CC 2015: The most linked pictures were shown in a correct way, but a few other pictures were shown as a grey box with a darker grey coloured cross in it. It was no bad set up in my settings in my case.
I found out that these pictures had a resolution of 1 dpi. It has to be 300 dpi, of course. After I corrected it with Photoshop, these pictures were shown in the right way in my InDesign...
Hope this solution helps...