There are 2 common reasons for that: 1) lack of the necessary codecs on your machine and 2) outdated graphic card drivers.
1) Try using a different video player.
You can use one that plays higher amount of formats and has its own embedded codecs (e.g. VLC or GOM player).
Else, please try installing the codec pack that allows your default video player reading another codecs.
The example of that is KLite Mega codec pack.
2) If that did not solve the issue, you might want to check if your graphic card is up-to-date.
Please follow this howto and then restart your PC.
After that, re-export the video.