unable to access Plesk login/ https site

Running Windows XP using Wifi to connect to the internet. can access some https sites but not the login for my website's control panel.
I've disabled my firewall/ antivirus, tried 3 different browsers, cleared my cache, have been on tech support (ha!) with my ISP and the webhost - both said there was no problem on their end, and still can't get in. I keep getting "You are not connected to the Internet.
The website is encountering problems.
There might be a typing error in the address." errors no matter what I do. Any suggestions - bearing in mind I'm not very tech savvy - would be greatly appreciated!