That sounds like it might just be an issue with yahoo email. I would wait a few hours and try again as maybe they are working on something. If you have mails they will show in your inbox.
If using a browser to view webmail, try a different browser like Firefox (or if using Firefox try IE). Trying a different browser will at least prove something in your browser isnt screwed up.
You can download Firefox here (all the browsers are free of course) https://www.mozilla.org/en-US/firefox/new/