The European Space Agency received no signals from the Philae lander during a scheduled effort to establish communication today.
Paolo Ferri, the ESA's head of mission operations, told the Associated Press that the orbiter did not pick up any signals from Philae on comet 67P/Churyumov-Gerasimenko.
Yesterday, the ESA performed a rotating operation to pull the lander out of a shadow so that its solar panels could recharge its depleted batteries.
Even if that operation was successful, it may take days or weeks until Philae's batteries are strong enough to send signals again.
"It is highly unlikely that we will establish any kind of communication any time soon, but nevertheless the orbiter will continue to listen for possible signals," said Mr Ferri.
On Thursday, Philae landed next to a cliff which largely blocked sunlight from reaching its solar panels.
The historic landing was the climax of a 10-year journey on board the Rosetta space probe. Since alighting on the comet, some 311 million miles (500 million km) distant from Earth, the lander has performed a series of tests and sent back reams of data, including photos.
Yesterday, the spacecraft was given commands to rotate itself to catch more sunlight and to drill a hole into the comet.
"We know that all the movements of the operation were performed and all the data was sent down," Mr Ferri said today. "However, at this point we do not even know if it really succeeded and if it (the drill) even touched the ground during the drilling operation."
Material beneath the surface of the comet has remained almost unchanged for 4.5 billion years, so the samples would be a cosmic time capsule that scientists are eager to study.
Scientists hope the 1.6 billion US dollar (£1 billion) project will help answer questions about the origins of the universe and life on Earth.
Communication with the lander has been slow, with signals taking more than 28 minutes to travel between Earth and Rosetta.
Scientists say they already have gathered huge amounts of data and are calling the first-ever comet landing a roaring success.
"Let's stop looking at things that we could have done if everything had worked properly," flight director Andrea Accomazzo said yesterday. "Let us look at things that we have done, what we have achieved and what we have on the ground. This is unique and will be unique forever."