OK, this is just an idea. I don't use the website myself (I download my mail through POP3), but it could be that you are now using the new Yahoo! Mail
beta, which is different to the old site. At the top of the screen, you should see 'Switch back'. Click it to go back to the old system, and then see if you can access your required emails. Come back to us to let us know how you get on.