No Text in Documents

Hi. I opened a project I haven’t worked on for several months and found text had disappeared from some of the documents in the binder. Likewise, under Research, binder links to websites are “blank”, i.e., they do not link. Re-starting the iMac has no effect and I cannot find anything in the Preferences menus that appears relevant.

Can you suggest what I might do to recover those documents and research links?