Yahoo have now put up a much better error page that goes a little bit further to explain the problem. They now suggest that you scan your computer for spyware and viruses (yup, the old "blame it on a virus" excuse!), and they also say:
This problem may be due to unusual network activity coming from your Internet Service Provider. we recommend that you report this problem to them.
This answer seems to hint at the bandwidth limiting system that I talk about above, where Yahoo monitor how much traffic your IP number has used and cuts you off after a certain amount of time.
that was from a site for yahoo problem but it might be right as I said before I would re-boot your comp and if that does'nt work re-install yahoo