You didn't really give enough information for us to offer a useful reply.

What happened to the contacts? Is the database corrupt? Or did someone actually delete all the contacts?

Is your mail hosted on a Corporate Domino server? Or is this a standalone client that you use to connect to mail servers (like gmail, etc) using POP?

Resolution:
Follow the given steps to resolve the error:
1) Restore the data from recent database backup
2) R fixup -f", then "compact -c -i" command to recover the corrupt database.

Note: If the inbuilt Lotus repair doesn’t help you to repair the corrupt database then, you require some alternative Lotus Notes Recovery software to repair and recover the corrupt database items. Or you can try to use repair nsf file lotus notes, very nice paid service !