Has the server for your domain MyDomain.com been upgraded to handle necessary changes in WordPress upgrades? How do you access a control panel for MyDomain.com? Are you trying to access your Yahoo.com small business hosting package?
You need to know what kind of hosting plan you have. Are you using their site solution, site builder, or another Yahoo or third party site building tool? Depending on your hosting package you purchased I will be able to help you find how to use an admin panel if it comes with one.
Also, I assume you contacted Yahoo with their #800 number? Either way I will try to help you find out where you can check status of your web site upgrade and how to access the control panel. You should have been given a username and password for your yahoo small business starter package.