Lost documents

All of a sudden when I use my word processor (MS Word) the menu in the upper left corner has stopped listing the four most recent documents. Normally, these would be listed at the bottom of the menu.

This is a nuisance both because it means extra steps when I want to work on something, and because it bothers me to have something not working as it ought.

Is this a common problem? I never encountered it before, but of course that doesn't mean it doesn't happen. And, more important, has anyone a solution?

Thanks.