It was a bad day for huge websites today, as both Wikipedia and YouTube experienced problems that made them unavailable for plenty of users for more than an hour. The issues at Wikipedia have been linked to an overheating problem at one of the European data centers, while Google has yet to clarify the YouTube issue.

"Due to an overheating problem in our European data center many of our servers turned off to protect themselves. As this impacted all Wikipedia and other projects access from European users, we were forced to move all user traffic to our Florida cluster, for which we have a standard quick failover procedure in place, that changes our DNS entries," the post explaining the failure on the Wikimedia Technical Blog read.

"However, shortly after we did this failover switch, it turned out that this failover mechanism was now broken, causing the DNS resolution of Wikimedia sites to stop working globally," it added.

The problem was thought to take no longer than one hour to be resolved, this is how long DNS servers should regularly cache an address, but the site was inaccessible for some people for a longer period. The site should now be accessible to all. The SSL gateway, secure.wikimedia.org, was disabled due to overload issues, but is now back up.

YouTube was also down earlier for almost two hours. The outage didn't affect the entire site, as individual video pages were still accessible and embedded videos worked properly. The homepage, though, was unreachable. YouTube has acknowledged the problem but hasn't provided any further details. The video site is now back up.

"YouTube is up again following a technical issue which has now been resolved," Google said in a statement. "We know how important YouTube is for people and apologize for any inconvenience the downtime may have caused."