How do you track down a fault in the DNS system?

I own a website that, until yesterday, was working just fine. Now whenever I go to that domain I’m redirected to another website. The website is a domain placeholder.

I’ve checked the settings of my computer, and of my router. They are set to correctly get DNS information from my ISP, then Google. I’ve also checked my computer for Malware, but the results came out negative.

I know it’s not a browser issue, as pinging the domain pings the wrong IP address and when I added the domains to my hosts file, the site started working just fine again. My girlfriend who is in another country at the moment has no problem accessing the site without hosts modification.

What could be going on? How do I track down where the incorrect DNS information is coming from?