It means your document has become corrupted. While you are still able to open and view the document, you need to rebuild it in a new document. Open a new document, with same margins, page size and 'compose to' printer as the corrupted doc. Remove all styles by running the script or by deleting all but [no style] one by one. Import the styles from your corrupted document into the new one by using the flyout menu in the Styles palette. Then group and copy all elements on each spread in the original and power paste (contol+Alt+V) into the new document. You may find the corruption is due to a damaged font or graphic, in which case you need to check whether anything in particular is at fault. Otherwise the document will not print certain pages (which narrows down where the corrupted element lies).
Another reason for a PM doc to get corrupted is the 'newsletter' syndrome, where you reopen last issue, delete old items and put in new ones. Over time, this will lead to corrption. It is better to set up a newsletter template containing all repeating elements, and therefore opening a new doc each issue, not rehashing an old one.
