Earlier Today I made a short Video of a Race that I run, I wanted to send it to Facebook and at the Time of doing this my Droid Maxx was on the Network and the Video would not Load up to Facebook said there was an error.. so I change my setting to Wi-Fi and tried to send the Video that way but it would not send and so I went back to my picture Gallery to Look at the Video and all it showed was a Gray blank like screen with the Play indicator in the center of the screen I went to tap it nothing happen..
So I powered cycled the phone but it didn't resolve anything it still showed the same, but what gets me is before I went to send it to Facebook it was running fine so can I assume sending a Video to Facebook that doesn't go threw because of Network strength isn't there the Video is Lost or is there a way to Retrieve it by some means..
Try downloading a different player like MX Player'
If that won't play it, it could be a corrupted file.
I'd try pulling it over to a PC via copy over usb or FTP if you know how, and running some form of fix program against the file based on its file extension.