I've had this happen a couple of times. I'm not sure why, but I fixed it by going to settings/applications/video and clearing the cache. Because it happened a couple of times, I wanted to fix it forever so, I downloaded MX player from the market...er...Play Store. It has ads on the settings and list pages, but not during videos. Still, it's an awesome player. It plays everything and I can adjust the size of the video, volume and brightness right from the screen. The volume and brightness controls are invisible on each side of the video, so just moving up to the right of the video, increases/decreases the volume. Doing the same on the left increases/decreases the brightness. Touching the screen reveals an icon where you can adjust the video size. I haven't had any issues since using this app.