NASA has lost contact with the ISS as communications failed during a software update. The ISS can still communicate with the ground, but only when it's over Russia via ground stations there.
There is no immediate danger to the vessel or the crew and instructions on how to fix the problem were relayed to the six astronauts aboard.
"This morning, at approximately 9:45 a.m. EST [14:45 GMT], the International Space Station experienced a loss of communication with the ground," NASA said in a statement.
"At that time, flight controllers in Houston were updating the software onboard the station’s flight computers when one of the station’s data relay systems malfunctioned," it explained.
"The primary computer that controls critical station functions defaulted to a backup computer, but was not allowing the station to communicate with NASA’s Tracking and Data Relay Satellites," it added.
The crew is connecting a backup computer to attempt to restore communications. For now, all it means is that the ground crew can't relay information to the ISS or the other way around.
UPDATE: The problem has been fixed.