One issue that may cause this is a firewall lockout.  This is now a required feature of all eCommerce rated hosting servers.  Too many attempts to log into control panels used for website maintenance will cause the IP address used to access the site to be blocked from the server.  Error messages generated under these circumstances may be obtuse, in order to avoid giving real hackers enough information to break through the blockage.  Whenever you have a connection problem, your best bet is to start by contacting your host. Most likely, they will need your current IP address of the computer you are working online. Go to http://www.whatismyip.com

A useful FREE service is http://www.dyndns.com once you have an account, ask your host to whitelist this dynamic domain name that reads the IP address of your computer and keeps it current. Most routers connected to a cable modem have the ability to setup dynamic DNS updates. Ask someone technical around you to check if your router currently supports dynamic DNS updates.

DNS migration is the process of changing a domain name's address assignment  from one Internet Protocol (IP) address to another. This process may last for days due to the distributed nature of the Domain Name Service, which is managed by a wide area network that spans the globe.  While migration should be avoided when possible, there are circumstances under which it is  inevitable.  This includes events such as moving from one host to another.  The reason for this is that IP addresses are assigned to hosts and rented, while Domain Names are owned by end users.. Thus, it is rarely possible to manage a complete move of both the IP Address and the Domain name when moving content to a new set of hardware.

Steps to easy DNS migration

1. 24 hours before you move, contact your old host and request they lower your DNS zone file to 300 seconds in the Time To Live (TTL)
   
   
2. Change the Read/Write access of /includes/configure.php from 444 to 644
   
   
3. EDIT: /includes/configure.php using a text editor.
   
   
4. FIND: define('DB_SERVER', 'localhost'); // eg, localhost - should not be empty for productive servers
   
   
5. EDIT: localhost with the NEW IP address of the remote hosts database server. <= Request this from your new host and ask them to allow your old IP address of your website to access the database server
   
   
6. FIND: define('DB_SERVER_USERNAME', 'your_user');
     define('DB_SERVER_PASSWORD', 'your_password');
     define('DB_DATABASE', 'your_database_name');
   
   
7. EDIT: Change the information that will match the new database settings on NEW host.
   
   
8. UPLOAD File
   
   
9. Change the Read/Write access of /admin/includes/configure.php from 444 to 644
   
   
10. EDIT: /admin/includes/configure.php
    
    
11. FIND: define('DB_SERVER', 'localhost'); // eg, localhost - should not be empty for productive servers
    
    
12. EDIT: localhost with the NEW IP address of the remote hosts database server. <= Again must be allowed to access new host's database server IP from your old hosting domain IP. They will look it up.
    
    
13. FIND: define('DB_SERVER_USERNAME', 'your_user');
      define('DB_SERVER_PASSWORD', 'your_password');
      define('DB_DATABASE', 'your_database_name');
    
    
14. EDIT: Change the information that will match the new database settings on NEW host.
    
    
15. UPLOAD File
    
    
16. Change permissions of /includes/configure.php from 644 to 444
    
    
17. Change the permissions of /admin/includes/configure.php from 644 to 444

It is HIGHLY suggested you ask your new host to walk you through these steps. If they did not allow your old domain IP into the firewall and access the database server, your site will cause errors.