Unable to open webs in MS FrontPage 2000

I get the following Microsoft FrontPage error box if I try to open a web in FrontPage:

“Unable to open web. Possible causes:
1. The web server may not have the FrontPage Server Extensions instaslled.
2. The web server may be temporarily out of service.
3. If you are connecting through proxy server, the proxy settings may be incorrect.
4. An error may have occurred in the web server.
5. Opening a web using ftp:// is not allowed.
OK”

Can you help, please?

Thanks