First fix you could try is clearing cache and sessions folders.
On ftp (if you have not ssh access) go to public_html (or whatever your site root is).
Find folder /var/cache and var/sessions/ and delete all inside but not delete that named folders, just folders and files inside them.
If this does not help it might be permissions issue. Fix magento permissions according to this
http://www.magentocommerce.com/wiki/...le_permissions or you might ask hosting to do this for you.
I hope this could help you.