This is their status :-
We are determining the cause and will provide an update shortly.
Update: we are defending against a denial-of-service attack.
We are defending against a denial-of-service attack, and will update status again shortly.
Update: the site is back up, but we are continuing to defend against and recover from this attack.
Latest status go to their website
ntivirus company Sophos explains what a DDOS is: "A denial of service attack occurs when computers bombard a website with requests for information".
"Typically hackers can control thousands of innocent users' computers centrally and command them to visit a site that they wish to flood with traffic - making it impossible for other internet users to get through. It's a bit like 15 fat men trying to get through a revolving door at the same time - nothing can move".
Maybe there's too many services provide on twitter coz this DOS attack... Hope it will repair soon.