Clear your cache and cookies. Your browser still has a copy of the old (pre-upgraded) site in it's cache. Hence, it is logging you out of the upgraded site.
Clear your cache by pressing <ctrl> and F5 at the same time, clear your cookies through your browser controls. This will fix it.
Clear your cache by pressing <ctrl> and F5 at the same time, clear your cookies through your browser controls. This will fix it.