Yahoo is in the process of rolling out Yahoo Mail Beta to all users instead of select participants, so that may be the cause of you being unable to access your account. I haven't had any problems today, but then again I've had Mail Beta for nearly a year so that might make me exempt to the transition. Aside from trying another browser or another computer I'd give it a day or two. (Over a month ago they released a major update to Yahoo Mail Beta and I had problems accessing my account for nearly a week, so this wouldn't be anything new.)

John