I have the same problem. In the left menu, next to iCloud a little loading loop keep turning round. The Contact application is "Not Responding" according to Activity Monitor. I tryied Force Quitting Contact and relaunching it, same problem. I tryied rebooting, same problem. I deleted all AddressBook preferences and data from the Library, and after it retreived all from iCloud, I got to the same problem. Any ideas ?
My Address book was not opening anymore and went to "not responding". Forced stop did not resolve the issue.
Most likely to be an icloud issue. Try switching off contacts in your icloud settings in the configuration icon of your Mac. Delete contacts as otherwise you will end up with all duplicate contacts. If you have any contacts stored on your mac, be aware that when switching contacts in iCloud on again, all will be uploaded into iCLoud. So if you don't watn, export and delete before switching back on.
This solved it for me.