Hey there ifta,
Simply use the regular "Sign in" option in the upper right-hand corner of the site, and the fields to enter your login information will pop up.
Deadfish, were you by chance using the Google social sign-in? We had an issue with this briefly and had to require users to re-enter their login information if they had their previous session saved. If not, is the issue still occurring for you? The login for the Tom's Hardware main page (tomshardware.com) is the same as the login for the Forum area (tomshardware.com/forum/) so it should work for you on either.
Let us know and we can help you get this resolved!