If you receive the message "Android.process.acore has stopped unexpectedly", an Android system process has crashed. If your phone's system software is Sense 4 or lower, follow these steps to restart it:

1. Turn off syncing.
2. Back up all your contacts to Gmail, exporting the data in .CSV or .VCF format.
3. Navigate to Manage applications > Contacts storage.
4. Tap on Clear cache and Clear data. 
   NOTE: This will erase all contact data stored on your phone.
5. Turn on sync again.

Phones using Sense 4.1 or higher must perform an APK fix. See Android.process.acore force close fix for all devices running Android 4.1 (Jelly Bean) for instructions and details.