It sounds like a "feature" Rick.

Something has probably gotten corrupted in the document. This happens more often than we would like to admit.

See if this happens when you create a new document.

Also, file a support request http://support.xara.com They will probably want to study the document to see what is making this happen.

In the mean time, if you can create a new document and add new pages successfully, then copy and paste your current site to the new document.

You can copy the entire page and paste the entire page in place using Edit > Copy Page and Edit > Paste Page.