A note posted on the site reads, "File transfers have been temporarily suspended while Napster upgrades the databases that support our new file identification technology. Keep checking this space for updates. Thanks for your support!"
It is not known when the service will resume.
Napster has been trying to stop users from exchanging copyrighted music files since a court slapped an injunction on the company in February. The company installed filtering software that blocked songs according to artist name or song title, but users found they could side-step the software by entering mis-spelt song names.
To combat this, Napster has started using new technology. Since last Thursday, users have been required to download a new version of the filtering software, which identifies files by their audio content.
However, a number of files have managed to slip through the software and the company has suspended the services while it "cleans up" the system.
While Napster struggles to find a foolproof way to comply with the court injunction, another online entertainment company, Aimster, has been the target of a lawsuit brought by the film industry.
The lawsuit -- filed last week by Columbia Pictures, Disney Enterprises, MGM Studios, Sony Pictures, Twentieth Century Fox, Universal City Studios and Paramount Pictures -- accuses Aimster of posing a threat to the film industry with its online service, which allows users to download and swap films for free.
Aimster claims that it cannot monitor nor control the material that its users swap because there is no central directory of files. The film industry, however, says that the company is involved in assisting piracy because it provides a hub and software.