Wikipedia is on blackout for 24 hours. The blackout code is delivered via JS from the domain meta.wikimedia.org.
To get wikipedia back online during their blackout append "127.0.0.1 meta.wikimedia.org" to your hosts file.
If you're not familiar with what a hosts file is, here is a short overview. http://onwebdevelopment.blogspot.com/2008/06/blocking-advertisements-with-hosts-file.html
Or look here on how to edit your hosts file: http://www.windowsreference.com/windows-7/edit-hosts-file-in-windows-7-windows-vista/
These are for windows, however if you're on Linux then it's a lot simpler - like most things Linux.
echo "127.0.0.1 meta.wikimedia.org" >> /etc/hosts
You need to be root to do this, so either do "sudo" or "su root" or "sudo su" etc.
Other workarounds to wikipedia blackout
If you have foxyproxy you can also use that to block meta.wikimedia.org specifically by specifying that it proxy to some blackhole.
There is a number of other ways to block meta.wikimedia.org and they should all deliver the same results.