Mark is the one who can help but you can help him help you by describing exactly what you are doing when you get the error. The more details, the better.
I just made another chart and it is working fine. But, if I try to open the previous chart that I made, it will not let me edit it. I still get the error message. Oh, well it's okay again!
It sounds like the other file is corrupted. If you have a backup copy of it, you might be able to rename the extension (or right click and open as PCStitch) and see if you can correctly save it.