This has happened to me with 1.7.5.6-2 as well.
Only with me the router stopped responding during a portscan from grc.com.
I could still access the internet, but the router itself didn't respond to either telnet or web-interface.
I had to restore to 1.6.5.3 before I got it to respond again. After that I upgraded to 1.7.5.6-2, restored settings, and all was back to the way it should be.
Royan