Could be many things, we should start with this...
"Try Disk Utility
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair. Disk Utility checks and repairs the disk."
Then try a Safe Boot, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, reboot when it completes.
(Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive.)
If what BDAqua suggests does not solve the issue, with Mail not running go to the Users folder on your hard drive; locate and open the folder named the same as your active account. In that folder locate and open the folder named Library, and in that folder open the folder named Application Support.
In the Application Support folder, locate and open the folder named AddressBook. IN taht folder, locate the file whose name is
and throw it away.
Empty the Trash and restart the machine. Then test.
When i deleted "AddressBook-v22.abcddb" it removed all my contacts. I finally got my contacts back and following BDAqua and Don Archibald's advice, I still have the same problem. When I delete a contact in Address Book 5.0.3 it bombs. I have been working this for 3 hours and am totally frustrated...help again?!?!