I was viewing contacts in kaddressbook (running in Kontact).

I pressed f5 (don't ask me why!)

All contacts (several hundred) disappeared.

Shutting down and restarting didn't bring them back.

I am sure they still exist, but I can't display them.

Version is 4.4.7 on debian.

It seems to be using a database in ~/.local/share/akonadi/db_data

I have a current backup copy, but I'm not sure what file to restore. Anyhow, I am guessing that restoring backup won't fix it, as I believe there is some database problem that prevents the contacts displaying.

Where should I start?

First, take a look at ~/.local/share/contacts - are the records still there? If so, I would rename the akonadi folder (so that it's easily got back if needed) and let a new login re-create it. You will lose, I think, any tags added to things, but not your real data. Does that help?

The other possibility is that you should go to the akonadi console and see if the resource still looks good. If in doubt, I'd remove the resource and re-create it. That _should_ get everything back including your tags unless you have actual corruption somewhere. Maybe you want to try that first.