There are Restorable and Extra lives in the game. Restorable Lives are restored within certain time. You can buy Extra Lives or receive as a gift from friends.
You can receive extra lives from your friends. To receive lives from your friends go to the Messages Tab in the Events menu.
Be generous! Always send lives to your friends and they are sure to send lives to you too. Besides, you can hire citizens who increase the max number of restorable lives and reduce the time for restoring your lives.