Sony Pictures Entertainment has been hacked, leaving many employees without access to its network.

Employees on Monday were shown an image of a skeleton on their PCs from a group called #GOP, or Guardians of Peace, with a message that threatened the release of "top secret data" if demands, which have not been released, are not met.

Sony has not released any comments regarding the incident apart from saying it is investigating the issue. The company reportedly shut down its network and advised employees a resolution of the situation could take from one day to three weeks. Employees have also been told to turn off computers and disable Wi-Fi on mobile devices.

A user from Reddit posted an image allegedly taken from a Sony computer screen, which reads "Warning: We've already warned you, and this is just the beginning... We have obtained all your internal data including secrets and top secrets."

Sony has had a history with hackers. In August, the company's PlayStation Network was taken offline by a distributed denial of service (DDoS) attack after being hacked by a group known as Lizard Squad. DDoS attacks are commonly used by hackers to overwhelm a system with traffic, rendering a service temporarily unavailable. Lizard Squad then targeted John Smedly, president of Sony Online Entertainment, via Twitter.

Only a few days ago another group reportedly hacked the PlayStation Network once again, stealing gamers' email addresses and passwords. Sony has denied the hack, saying the released login information was fake.

"Three years ago, the hack on PlayStation network was massive, expensive and absolutely embarrassing. This time round, I don't believe that there will be massive damage, save for Sony's ego, even if the hack is real," said Wee Teck Loo, head of Euromonitor's consumer electronics research.

The news comes just a day after an advertisement for the PlayStation Vita was pulled from Sony's European YouTube account amid accusations of being sexist and misogynist. The ad depicted a female British voice apparently making allusions to masturbation, but was about gaming.

It is unclear how severe the latest hack is or if it the threats are empty. The hack does not seem to be at the same magnitude the company has suffered in previous attacks. Despite this, the fact that hackers have managed access once again to Sony's networks does not restore consumer confidence.