Have you tried to access your Yahoo account by using the browser on your phone and going to Mail at Yahoo.com?
What version of the Yahoo app are you using? The current 1.3.7 seems to be buggy and there are many complaints on the Market. I had some notification issues. Luckily I had backed up 1.3.6 on my SD card. When I re-installed that version Yahoo was working properly for me.
On the other hand, some complaints lead me to believe that Yahoo is having some account or server issues with some of their customers. If so and you are one of them, you will just have to wait for Yahoo to fix the problem. I have had some account issues with both Yahoo and Gmail in the past. Scared the poop out of me for a couple of days, but eventually things got fixed.
