Twitter is currently trying to battle a denial-of-service attack, with the website announcing the problem on Status blog.
The note explains: "We are defending against a denial-of-service attack, and will update status again shortly."
It continues: "Update: the site is back up, but we are continuing to defend against and recover from this attack."
As Graham Cluley from Sophos explains on his blog: "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."
Don't underestimate impact
He also notes about what this means for Twitter: "Don't underestimate the impact an attack like this can have, by the way. Twitter isn't just about meaningless piffle (although there's a fair bit of that).
"Companies are using it to keep in touch with their customer base, and consumers take advantage of the site's intimacy to get an answer from large companies that are discovering how to have a 'human face' online."
More when we get it.