I had trouble with their site the other day. I was on a newer comp and had to add Google's DNS servers, which made it show up instantly. Their servers are 8.8.8.8 and 8.8.4.4 and I would suggest adding them to your network connection to see if it solves your issue.
It's been a odd week with me & xenForo.com The site doesnt work for me at night, but during the day it is fine.. ;\
There was a few issues with their DNS servers, including them changing DNS providers & the old ones not responding to queries. Liam