I host a couple of websites on the same Ubuntu server, and as of today both domain names have gone down. PuTTY and FTP still both work so it is obviously a problem with the domain name.
I haven't made any changes with my registrar (namecheap), or my DNS host (clouDNS) which has both domains pointed to my IP address under the 'A' record.
The only other solution I can see must be some setting on my server, which is strange as I haven't changed anything there. I seem to recall editing a config file in order to set up the domains, but I cannot remember what it was at all!