The format in which the video was captured (the default is MPEG4) may not be supported by your current media player. You can try changing the capture format (while in camera mode, tap the screen followed by the Settings Icon>Advanced>Capture Format) or update your player.

Or you can check out this for help:

http://forums.wireless.att.com/t5/AT-T-HTC/Tilt-video-viewing-problem/m-p/1426954