i bet when they did the upgrade, the did not set the session.save_path variable in the php.ini file to enable sessions on your site.
to test this, use the sessionTest.php file from the following thread:
showpost.php?p=23826&postcount=2
if the session test fails, you will need to contact the host to have them correct the problem in the php.ini file by setting the correct session.save_path


