The two most common causes of PrE to not launch are:
An outdated/obsolete, or corrupt video driver. See this article for more info: http://forums.adobe.com/message/5176500#5176500
VST's that cannot load into PrE. For more detail, see: http://forums.adobe.com/message/3077222#3077222
It might be helpful, if you give us the full details of your computer and OS.