When I replace with after effects composition, in AE it's placeholder instead of my video?

To make this easier for everyone, I'm inserting your screenshot [below] so that others don't need to download the image onto their computer. That's strange why the .mp4 file doesn't journey over through dynamic link. It looks like it's just offline. What happens if you reload/replace the file with the original .mp4? Does any error message come up? Or does it simply relink?