The International Space Station lost communication with ground controllers on Tuesday but its commander says the crew is fine, NASA said.
It blamed a problem in a data relay system.
NASA said flight controllers in Houston were updating software on board the station's flight computers when one of the station's data relay systems malfunctioned.
The primary computer that controls critical station functions defaulted to a backup computer, but it was not letting the station communicate with NASA's Tracking and Data Relay Satellites, the US space agency said.
Officials in Houston were able to communicate later with the crew as the space station flew over Russian ground stations. They instructed the crew to connect a backup computer to begin the process of restoring communications.
The International Space Station marked its 10th anniversary of continuous human occupation on November 2, 2010. Since Expedition 1 in 2000, the space station has been visited by 204 individuals.