Well, could be few things, but first that comes to mind is that your microSD card might be corrupted..Try the above suggestion, format your card and then copy all your files back up..
Second, the movie (dpg file you're trying to play) itself is corrupted..you should try playing some other .DPF files to see if it's problem with Sakura or that one DPG movie..
Third, it could be the movie itself, I have a bunch of movies, freshly turned into DPG, and most of them work but two or three don't want to play, but freeze my Sakura....I don't know why that happens, I'm guessing it has something to do with the codecs and encoding process..so I just delete that movies..