Sometimes Internet cafes will block the http://www.irctc.co.in/ website (presumably to make you book through their related agent!)
You can get around this by typing in the IP address for the web site, i.e. http://203.94.240.82/ which will bring up the same website and you can then use it as normal.
(IP address may change over time; if it’s wrong, type “ping www.irctc.co.in” in a command window to get the right IP address).