The website was subjected to a 'denial-of-service attack', Twitter's status blog said.
In a denial-of-service attack, a company's servers are flooded with huge volumes of unwanted data in a bid to disable them. This typically happens when hundreds of remote-controlled virus-infected computers contact the site at the same time.
It is not clear when the attack exactly took place on Twitter, but it appears to have suffered a global blackout.
Although there is no clue yet as to who is behind the attack, Twitter said that it is trying to find out the reason and the people behind the blackout.
Twitter's status blog had initially said that the site was down, but later added: 'We are defending against a denial-of-service attack, and will update status again shortly.'
The company's growing popularity has led to many a malware maker turning their ire at it, and people believe that this could be the work of these malware makers.
