Right password and username still no sign in

clear all the cache and check the firewall..might be there is some issue in blocking some pages by your firewall

This is a longshot, but maybe some yahoo.com pages are being blocked by your computer. Do a search for your Hosts file and open it with notepad. Any listings that are prefixed with 127.0.0.1 are blocked. Look through these listings for anything with yahoo.com in it. If you find any, make a copy of the hosts file with a name you'll remember, then delete the entries containing yahoo.com. Save and close the Hosts file. Try to sign in to your yahoo accaount. If this doesn't work, use the copy you made of the Hosts file to restore the file.
Hope this helps.