Yeah, they currently suffering from DDoS , at least that is what it said at http://blog.libertyreserve.com/ .
Check here for more information: http://en.wikipedia.org/wiki/Denial-of-service_attack You will get your answer. I am sure this site will be up soon as they are one of reputed one.
That's very bad if there was a DDOS Attack. Hope they will appear again and there won't be any problems.