If you spend much time messing around the Appearance settings in the MythTV Setup pages, you run a good chance of eventually screwing things up to the point where MythFrontEnd will no longer load. This is especially probable if you start changing the Paint Engine and/or the QT Style settings.
If you've made changes and now you only get a black screen when you issue the mythfrontend command, it's likely that you've hashed up the config in some way.
Luckily, there's an easy fix for this one. Open up a terminal and run mythfrontend –reset and then run mythfrontend again.
The first command, resets MythFrontEnd to the default Appearance settings, but it does not actually execute mythfrontend. When you run mythfrontend for the second time, it should start right up with all the default Appearance settings.