"We've already warned you, and this is just a beginning," the message said. "We continue till our request be met.... If you don't obey us, we'll release data shown below to the world."
Sony instructed employees to turn off their computers and disable Wi-Fi communication on their mobile devices, Variety reported.
Sony told the LA Times it was investigating "an IT matter," but it did not disclose further details.
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The attempt is the latest in a string of attacks against major corporations. Millions of customers' card and personal data were compromised by recent data breaches at JPMorgan, Home Depot and Target. The Target breach alone affected nearly 40 million consumers.
Sony's PlayStation unit was targeted by cyberthieves in 2011. That attack compromised more than 70 million online user accounts.
PlayStation was also hit by a denial-of service attack in August that overwhelmed the system with fake network request.
A Sony representative did not immediately respond to CNBC's e-mailed request for comment.
In what appeared to be an automated response, the company said "Our email system is currently experiencing a disruption. Please contact the person you wish to reach via office or mobile phone."