Access problems.
ESTABLISH FIRST
Log in to what: cPanel, their site, email, or your billing system?
Each is different.
FOR CPANEL
Check the username, the password, and whether the account is active.
Reset the password if needed.
FOR THEIR SITE ADMINISTRATION
That is their application, not hosting.
You can reset it through the database or a tool, if that is within your support scope.
FOR EMAIL
Check in webmail first. If webmail works, it is a client configuration issue.
WHAT TO CHECK
Whether the account is suspended Whether they are blocked by failed attempts Whether they are using the correct address
THE BLOCKING POINT
Repeated failed attempts can block an address temporarily.
Check, and clear it if appropriate.
WHAT TO VERIFY BEFORE RESETTING ANYTHING
That the person asking is the account holder.
THAT MATTERS
Resetting access for someone impersonating a client is a serious failure.
Verify through your records, not the details in the request.
WHAT TO DO AFTERWARDS
Require them to set a strong password Suggest two-factor authentication
