Have a have a Windows VPS using Plesk 9.
Until today I have been using the http://myIPaddress:8443 method of accessing Plesk Panel but now I can't access via a browser.
This is the Google Chrome error (Error 101 (net::ERR_CONNECTION_RESET): Unknown error).
If I use an RDC connection and open a browser window and connect via http://localhost:8443 it works just fine.
What could have gone wrong?
The ERR_CONNECTION_RESET indicates that the communication between your browser and your server is cut short, in effect, your browser is asking your server "What's up!" and before your server can respond, the communication drops.
This can be several things. It could be the firewall on your server. It could be a firewall on your internet connection and/or PC or it could be your ISP itself.
If you have the ability to try a different ISP (maybe even from a different PC, like an internet cafe) to test that it's not on your end, you can eliminate any software on your PC or your own network from this almost instantly.
Using RDP and being able to access your Plesk installation on the server itself indicates that there's nothing wrong, it could be as simple as just a piece of software you installed or setting you changed by accident. (Peerblock sometimes block's communication that's legitimate)
The last thing you can check is (if you're running IIS, which I assume you are) to check on the website settings which IP addresses, for that installation, is allowed. It could be that it's set to 127.0.0.1, by default I'd keep it to listen on all IP addresses if I were you.
Otherwise, if you've checked from a different ISP and computer and you're still not able to access it, I'd recommend submitting a support ticket with your hosting company to double check the installation.