Tell us more. It was there and suddenly it disappeared? Was it after an upgrade? Look for it by clicking help>moretroubleshootinginformation, scroll down to Profile, click 'open folder' and look for a file called abook.sqlite. That would be the addressbook. Somehow, it's not being accessed. Note the location of it on your PC. To reinstate it, you might try this (although I haven't tested it): - click tools>import>File>addressbook>SQlite database and then select the one you saw.

I have no idea whether it will reactivate your addressbook, but it's a try.