Re: Lost all contacts on I mac after downloading yosemite
Are the contacts associated with an email service like iCloud, Gmail, Outlook.com, Yahoo Mail, etc.? If so, logon to the email account via a computer to see if the contacts are present. If they are, then simply reconfigure the email service on your Mac and/or go to System Preferences - Internet Accounts - [your email service] and make sure that there's a checkmark next to "Contacts" so they can sync to your Mac. If they are not present online, and if you do not have a backup .vcf file or similar of those contacts or a Time Capsule backup containing the contacts then consider them lost.