1. Don't take this the wrong way but what were you doing that might have lead to loss of the contacts?
2. Do you have a backup of your data?
3. Are you using anything like iCloud that syncs data across computers/devices?
1. This was the first time I had tried to use contacts after upgrading to Yosemite two days ago.
2. No. (Yes, I know I am an imbecile.)
3. A local Mac Experience person said on the phone that I should add iCloud to my accounts and that somehow I might be able to recover the data that way. So I did. Don't know what (if anything) I do next.