I can't remember exactly what the question mark does but when you type in the address
www.zerogain.com/forum in the browser it then looks under the .com DNS (domain name server) which is a large database of domain names and their IP addresses to the servers the website is hosted on. So when you type in
www.zerogain.com it looks up the DNS server and finds the IP address for the server.
Some times I am not sure why when you type a URL in the it dosn't get the information from the DNS server properly thus the browser dosn't know the IP address so cannot display anything.
When you place a question mark after the URL it forces the browser to look up the DNS server again, where is if it you just retyped it the DNS is data will be stored in your cache on your hard disk so may not work correctly.
Thats the easiest way I can explain it, its a more complex than that, and what I said above my not be quite true, please can somebody confirm that.
I am not sure why these erros in the first place happen though, I would think its an ISP problem. Also many on many servers you can block IP addresses (this happened to me once before, I was banned from a website as a joke).