Take all those files and put them under a new folder on the existing website which is working.
Try to open that folder on browser and check the index file.
If the files are working that means the files have no issues.
Next thing, go back to your control panel and see if you have two index files.
Delete one of them.
Also check in CPANEL if the files are at the right directory.
You also need to check if your nameservers are configured correctly in the domain registrar.