As of now it is still unsure what exactly happened, but the popular Dutch news site nu.nl reports that the Canadian Recording Industry Association (CRIA) is responsible for the downtime. Websites such as TorrentFreak contacted some of the Demonoid administrators, but they are not sure what happened either. It is certainly possible that Demonoid's Canadian ISP pulled the plug after being pressured by the CRIA. The Demonoid server is still pinging, but the ISP could have firewalled the everything after they received some serious legal threats. Deimos, the founder and the head admin of the site is unreachable and has not responded yet.
This is not the first time Demonoid suffers major downtime due to pressure from the anti-piracy lobby. Demonoid had to move its servers from The Netherlands to Canada in June after The Dutch anti-piracy outfit BREIN filed a subpoena against Demonoid's ISP. BREIN had asked the ISP to take Demonoid offline and hand over the administrator's personal details, but Demonoid relocated their servers before any harm was done. Unfortunately, it now seems that Canada is not the "safe haven" as they expected it to be.