According to users from all over the world, the short message service Twitter has been having access problems for hours.
The Downdetector platform, which specializes in Internet failures, reported a significant increase in problems on Twitter on Thursday night.
Users would no longer have been able to see their notifications and no longer be able to use other functions.
It was one of Twitter's worst technical disruptions since Elon Musk took over.
At the height of the disruptions, Downdetector reported more than 10,000 complaints across the United States around 5 a.m. Central European Time.
In other countries, according to the platform, the number of complaints ranged from a few hundred to several thousand.
The cause of the problems was initially unclear.
There was some controversy after Musk's takeover
Using the keyword "#TwitterDown", users themselves reported numerous problems in the online service.
"Can anyone see this, or is Twitter broken?" one user wrote, only to receive the reply from Musk himself, "It works for me." Musk, who also runs electric car maker Tesla, bought Twitter for $44 billion in October ( 41.4 billion euros).
The takeover has been overshadowed by numerous controversies, for example because Musk immediately fired the top management of the online service and then around half of the workforce.
According to industry insiders, this is likely to affect the online service's ability to quickly fix technical problems.
After 57.5 percent of participants voted for Musk's resignation as Twitter boss in a poll he initiated himself, the billionaire agreed to this step last week.
"I will step down as CEO as soon as I find someone foolish enough to take the job," he tweeted.
He will continue to lead the software and server teams at Twitter.