My comments disappeared!

I'm using Acrobat 9.5.5 on a MacBook Pro running OS 10.10.3. The editor is also using a Mac.

When I opened the file I'm working with, all of the comments were gone. And I can't find the menu or toolbar spot for turning them back on. Everything was fine last week, the last time I used this document. What in the world can be the remedy?

Thanks,

Nicky