How long is your entire project and how long is the exported MP3? Is it really just one sample long (basically meaning just a tiny fraction of a second) or is it one particular audio object from your arrangement that gets exported. Make sure you haven't accidentally selected a range in the project and ticked the range object option in the audio export dialog. That's the only thing that comes to mind right now. If that doesn't help, maybe you can provide a few more details about the specifics of your project.