If Yahoo! Mail will not open it is most often due to a browser issue, an incorrectly entered username or password, or because of a problem with the Yahoo! servers. To start, try clearing the cache and reloading the browser page, then verify that the correct username and password are being used. If still encountering trouble, it's best to contact Yahoo! customer support for additional troubleshooting.
Users can contact Yahoo! support directly about login issues. The company offers online help on its website, as well as telephone assistance at 1-800-318-0612. Yahoo! Help covers topics like forgotten passwords, accessing email on mobile devices, dealing with locked accounts, restoring settings and telling if an account has been hacked.
If it is a browser issue and clearing the cache did not help, check to ensure that the browser has been enabled to allow use of Yahoo!. To adjust permissions with the Firefox browser, type the phrase "about:permissions" in the address bar. This should bring up a list of websites on the left side of the page. Find the list of Yahoo! servers and manually remove the cookies from each one. Note that other browsers may have different procedures for cookie removal.