Try shutting down your computer and leaving it shut down for 5 - 10 minutes. I have had massive problems with Yahoo mail and have switched anything important to a different mail service. My account has been used twice to phish everyone in my address book. The same tactic was attempted in either my gmail or hotmail account and they (the email service provider) nipped it in the bud. I suggest changing your provider when you get back in. Take your time, but do change.

I have had problems getting into Yahoo off and on for the past week or so. I only use it for shopping. Switch to gmail - it's great!