I would first ensure that your java is up-to-date by visiting:
Download and install the latest java which is appropriate for your operating system, then restart your PC.
I would also recommend clearing your internet cache and cookies, by downloading and installing "CCleaner":
When installing the program, be sure to uncheck any "extras" it tries to offer you. Start up CCleaner, and run a cleaning on your computer.. being sure that it is set to clean internet cookies, cache, etc.. Make sure that you don't have any web browsing programs (Internet Explorer, Google Chrome, Firefox) running!! Run the process. Once it finishes, close the program.
Start up a web browser. I recommend Google Chrome or Mozilla Firefox.. (NEVER Internet Explorer!!)
Try to access your yahoo email. If it works, great!! If not, I (like most of the other answers here) would HIGHLY recommend that you just do the wise and safe thing.. and create a gmail account.. It's free, and FARRRR more safe and secore than yahoo.
I hope my answer helps!