Moved Site To New Server, WP-ADMIN.PHP not working HELP

I recently moved a site from my server to my clients. Everything works just fine and dandy.

Only problem is when I try to access wp-admin , which is kind of important for my client.

I’ve cleared caches and cookies, I’ve used different browsers, to no avail

Should I replace the wp-admin.php file? If I do that, could bad things happen?

your speedy assistance is ever so appreciated 🙂