Can't get online - Yahoo Mail

Use a proxy server,
some internet provider block certain websites. you can usually fix this by using proxy server.

http://www.hidemyass.com/proxy-list/

that website offer proxy servers for free.

if this website is also block try this servers.

208.92.249.118   port 80

66.197.164.213   port 80

157.95.56.15       port 80

Web browser instructions
Mozilla Firefox: Tools > Options > Advanced > Settings > Manual proxy configuration.
Google Chrome: Tools > Options > Under the bonnet > Network > Change proxy settings > LAN settings > Use a proxy server > Advanced > HTTP.
Internet Explorer: Tools > Internet options > Connections > LAN settings > Use a proxy server > Advanced > HTTP.
Opera: Tools > Preferences > Advanced > Network.