Cannot add pages to menu

For years I have been creating a table in Table press (latest version), adding the table to a page then putting it in the menu of the twenty-ten theme. All of a sudden I am unable to add any pages to the menu. I tried deactivating plug-ins and that didn’t help. I did not change any code.
I keep getting a “page not found” error. I have even tried adding pages that were already in the menu, and they gave me the same error message. Any suggestions out there. ?