Check the config.inc.php file and see if your user name and password are there.
It could also be that the server is handling sessions differently. Try the tester script from the Free Download page and see what the test results show.
To me, this sounds like your hosting company moved your account to a different server and your DNS is now pointing to a different IP than where you're uploading your changes.
Your host has definitely made some changes that is adversely affecting the execution of the calendar script. Do you have any other PHP scripts (names not required) that are functioning properly?
If you can allow me FTP access to your server, please open a Support Ticket and I'll take a look. http://phpcal.com/support/