Hi Neil,
I'm a bit disappointed you didn't give the version of PD you are using. As both you and Tricia have reported the "same" event it would be really good to know IF there was a bug that is causing this and IF it was being replicated in your systems.
A PD crash may be preventable if you'd applied the latest patch - a previously corrected issue.
Use "Save Project as" - this is what WE all do. and save to a different pds file.
Overwriting a complexed saved pds is fraught with likely issues. You have basically learnt a valuable editing lesson - save to a new file and always have a back up/fall back.
Opening a pds with either wordpad or notepad can, if you know how to, allow you to add or alter the pds' script. In your issue here I really don't know if it will help.
Look up:
http://forum.cyberlink.com/forum/posts/list/2405.page
Subject: PowerDirector - information we request and tasks to carry out.
PART O
Dafydd