One issue we have seen a couple times is that periodically when running on a network project, AutoCAD Plant 3D may hang at the startup screen.
Thankfully, there is an easy fix called the PnPLocalDataCachePurger.exe. The program is located in your installation directory:
“C:\Program Files\Autodesk\AutoCAD Plant 3D 2012 – English\PnPLocalDataCachePurger.exe”
A blog article by Mangesh Kalbhor gives more information on it:
You may want to put a shortcut on your desktop for the LDC Purger.