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If there has been a DNS change, it will take a couple days before you can view your site as it was before. In most cases, this will clear up within 24-36 hours up to 72 hours maximum.
You could alternatively use /your.ip.adress/~username (or what ever root directory you have for now until it's resolved) and then change back to yourdomain.com (mainfile.php)
You need to empty your /templates_c folder and if you use the module caching feature you need to delete the files in /cache as well, but most likely you won't need to. Main thing is to empty the /templates_c folder as tis will contain old IP information in the files.
Double check permissions (CHMOD /uploads, /templates)c, and /cache to 777) in case they were not carried over to the new IP address.
Most of these issues will clear up if you check what I have suggested above, mainly your /templates_c directory.