Error moving IP address to Domain Name

Background of what I have did.
I am running on a cloud server which uses Linux. I set up a apache virtual hosts base on this website.
https://www.digitalocean.com/community/articles/how-to-set-up-apache-virtual-hosts-on-ubuntu-12-04-lts
Before I change my IP to domain name everything works fine, after that I map my domain name to my IP on godaddy.com (I am able to ping my domain) and then from my http://IPaddress/wp-admin on the general panel I changed both my wordpress URL and site URL to (Yourdomainname.com)
It work immediately for a few hours but suddenly the website went down. In google chrome it says “This web page is not available” So I was wondering what went wrong as I did not touch anything after it’s running. (Research on many methods and none work)
So I decided on my second try:

1: Search and replace string on wp_posts changing from ip address to yourdomainname.com

2: http://codex.wordpress.org/Changing_The_Site_URL using FTP method I have put the code for functions.php in my theme.

But when I refresh myIPaddress/wp-login.php and log in it will redirect me to yourdomainname/wp-admin.php which is another “This web page is not available”

So how can I solve this error and let my webpage to be up and running again?