Unable to open a particular website on the network. HELP

Hi there, I am unable to open a particular website on my network. I was able to open/browse this website until two days back, but now since today i have not been able to open this site on my network.

This network is my private business holding, with 12-15 computers connected over wifi or some wired aswell.

None of the computers/laptop is able to view this site. But when i try to open this site from my home, or other places i am able to view it.
This does not work only in my office network. I checked my router/firewall admin page to see if the site has not been blocked etc. But I dont see anything like that.
None site is blocked/filter in the firewall page.

Please help. I am unable to understand anything.. how to make this work. The site is our company homepage, hence we are not even able to check our emails over outlook express.