Hi,
I suspect it has to do with your internet settings. Have you recently changed computers or updated your browser? Your browser must be able to accept cookies (little identifiers) in order to keep you logged into the forum. If your browser is not 'remembering' where you have been, then that is probably what is going on.
On IE check Internet Options and security settings.
On FireFox check Tools>options>privacy
Cheers,
BD