The PlanAhead tool hangs when the test bench is modified externally and the ISIM is relaunched.
If the PlanAhead window is minimized and opened again, it becomes a black screen and it cannot be terminated. The only way to terminate it is to kill it from the Task Manager.
One way to work around this issue is to use the built-in PlanAhead text editor.
This issue is resolved in the 14.5 PlanAhead tool.