wp-admin page not found after reverting to previous version

I just tried to upgrade to the latest version of WP but had some issues with my sidebar widgets in the new version so reverted to the version that I had backed up just prior to attempting the upgrade. The front end of my blog is displaying fine again but when I attempt to log into to my my wp-admin I get “Internet Explorer cannot display the webpage”. Can anybody suggest why this might have happened and how I would be able to go about logging into my dashboard now?

Thanks in advance for any assistance – I’m a bit desperate!!!

Sarah