1. Post your phone information (model & firmware)
2. Search this forum for previous posts on the topic
3. Turn off and then turn on your phone again before you test your video - it may start working again.
if hgohel's suggestions (specially the last one) do not bring any fruit to you, then do a factory resetting (*#7780#).
Alot of people are having issues with RealPlayer not to play the recorded videos or playing partially (only the sound can be heard).