For those who can't get in at all try this as it seems to be working for a few.http://myyahoo.com
Login on My Yahoo! page
Select the link at top right to go to Yahoo!'s Home Page.
Mouse over the Mail Icon under where it says (Hi, your Yahoo! id).
You should get a drop down menu with your latest emails displayed.
Select an email and click it. You then get to a new page about Shortcuts, but just select the button for Mail and it should take you to your Mail page.
It wasn't getting redirected to the shortcuts page on some servers and not going to the mail page.
This is also working for some:
Click the "Options" link in the upper right of your Mail page.
On the Options page, in the "Management" column, click on "General Preferences
Find the "Composing E-mails" section, "Mode:", and choose "Compose messages as plain text".
Scroll to the bottom and click the "Save" button.
Now try it. Can you Compose, Reply to a message or Forward?
This is working for some also:
Make sure your screen resolution is 1024 by 768 pixels. Right click on your desktop, click Properties, click the "Settings" tab, left click and hold on the screen resolution bar and drag it all the way to the right, click Apply and click OK.
Please make sure that you are entering your correct username and password.