HOw do i reset my icould and basically verything on my mac, because since i downloaded some app, it has messed up my email and my contacts

Can someone please help me with this, ever since i downloaded this alternative contact application, my emials that were synced with my mac have been all messed up, as well as my contact list. My contacts have not been deleted, but it wont let me add contacts past a certain number of cards already in the list, as well, some of my contacts just dissapear and i must re enter them.

Any help will be appreciated,

Thanks