quickly and ask whether they have backups. Yes - you may have to pay, but this may be something money can't buy. There may also be a 'ghost' copy somewhere on your HD. Enable "view all files" and use "Search" to find all files which contain some unique word in the address book. This is a very, very long shot.
Somehow, while creating groups, I have deleted an entire yahoo address book with over 600 addresses in it. Any ideas on if there is ANY way to get this back? As they say, money is no object...lol