No webpage when accessing Yahoo Mail ?

To me this sounds like files are corrupt in cache. I suggest a clean up of your old cache and cookies.

To delete cookies
To delete a single cookie
1) In Internet Explorer, click the Tools button, and then click Internet Options.
2) On the General tab, under Browsing history, click Settings.
Click the View files button.
3) Click the Name column heading to sort all the files alphabetically, and then scroll down until you see files that begin with the prefix Cookie:. All cookies will have that prefix, and they usually contain the name of the website that created the cookie.
4) Right-click the cookie you want to delete, click Delete, and then click Yes.
5) Close the window that contains the list of files, and then click OK twice to return to Internet Explorer.

To delete all cookies
1) In Internet Explorer, click the Tools button, and then click Internet Options.
2) On the General tab, under Browsing history, click Delete.
3) Select the check box next to Cookies. If you only want to delete cookies, clear any other check boxes.
4) Click Delete, and then click OK.
NOTE:
Some websites store your user name, password, or other information about you in a cookie. If you delete that cookie, you might need to enter your personal information again the next time you visit the site.

If that doesn't work then you may need to update your Java and Flash Player.

Updating Java
You can download the latest
Java here.

Updating Flash Playe

it can be found herehttp://schematicsdiagram.blogspot.com/2011/06/how-to-update-flash-player.html

After you have tried both, restart you computer and see if you still have the problem. If so then I suggest you run a full AntiVirus scan of your system. I have compiled a list of the best FREE AntiVirus programs on the net, it can be found here: http://schematicsdiagram.blogspot.com/2011/06/free-anti-virus-software.html