Hmm weird indeed. I know there was a problem yesterday (perhaps the day before too) I couldn't reply or send a PM to anyone. That seems OK now though.
Stupid question, have you checked the 'remember me' box?
Also (as any tech will tell you
) ) try clearing all your cookies/cache. That usually works for me. I'm sure there was some sort of problem - what with the slow site and my PM problem - maybe they have fixed it now.
Hopefully you will be able to use it properly soon.