The US Senate contact site is facing technical difficulties, likely due to the large incoming amount of e-mails regarding the controversial SOPA bill. Many sites, including Google and Wikipedia, posted some form of protest against SOPA on Wednesday in order to raise public awareness regarding the issue and urge them to do something about it.
There is no definite proof that the SOPA backlash is responsible, although sites have made frequent appeals to their visitors to call or e-mail their senators. A hack attack is also unlikely, as even famous hacker group Anonymous is under a blackout.
