I lost a great amount of my contacts. How do I retrieve them back?

Contacts “belong” to email accounts; each email account on your phone may have associated contacts. Go to Settings/Contacts/Accounts and see which accounts have Contacts enabled (it will say “contacts” below the account name). If you delete an email account any contacts associated with it will disappear; likewise if the account stops working.

If you look in Settings/Contacts there is a “default account”. This is the account new contacts will be saved to.