Hi,
Very hard to say without more details.
I can offer a couple thoughts. I will assume that you’ve copied all files over from the old site, and made sure that their file permissions are still correct. I will also assume that you have loaded the database to the new site.
If that’s true, here’s a thought. Frequently you cannot use the identical database name on the new site. Their account might force an extra prefix with their account name in it, for instance. In that case, you’d need to change that name in manager/includes/config.inc.php
All kinds of other possibilities, though, based on exactly what the error message was, what kind of server you’re on, actually all the stuff that posting on here prompts you to enter, hint, hint.
good luck.