How to fix the error Unfortunately, the process "android.process.acore has stopped"?

I got the solution. By mistake I removed the Contacts app. Every time, Contacts Storage app is checking for contacts.So Now again I installed Contacts.apk. And it works fine after installing.

one more solution is

   Settings --> apps --> All --> Contacts Storage --> clear data and Force stop 

Backup your contacts. Go to settings and clear the "Data" of Contact Storage. This will remove all your contacts. After that, restore your contacts back and the error will go. This basically happens when FB messes up with your contacts.