Sometimes, this error goes by just closing, relaunching and re-exporting, but in your case it seems that your project is having some kind of corrrupt files in or around 50 % of your timeline.
1) Make sure that you don not have a blank space in your timeline, zoom into your timeline to verify that. If for some part of your video you want the video to be blank that insert a black matte, instead of blank space in the timeline.
2) Re-position your work area bar, so that it covers approximately the part of the timeline from 40%-60%, and then manually render by pressing Enter to see, if the render completes succesfully
3) Cut and paste the area that approximately covers the part of the timeline from 40%-60%, into a new project and then check to see if that exports succesfully
Find the corrupt part using above steps and then remove the corrupt file or correct the timeline
Your not really telling us much about your PRE version, PC details (processor, hard disks, drivers, graphics, sound), project setting, project assets (videos, still, sounds) where and how you got them, and how you got them into PRE. Take a look at these:
Insanity is hereditary, you get it from your children
These messages are not necessarily about PrEL, but read for the ideas
Long File Names or odd characters cause problems
And #4 Here http://forums.adobe.com/thread/666558
And This Message Thread http://forums.adobe.com/thread/665641
Photo Scaling for Video http://forums.adobe.com/thread/450798
-Too Large May = Crash http://forums.adobe.com/thread/879967