A notice on the National Aeronautics and Space Administration's Web site says Spirit radioed a beep Thursday morning, which shows it can still receive commands from Earth, but has not transmitted any data since early Wednesday morning.
"Flight-team engineers for NASA's Mars Exploration Rover Project are working to diagnose the cause of communications difficulties," the notice said.
NASA has said its team can fix software problems from Earth, but a hardware problem would be far worse, said project manager Pete Theisinger.
"This is a serious problem. This is an extremely serious anomaly," he said.
Since Spirit's Jan. 3 landing in the Gusev Crater on Mars, project managers have been carefully testing its instruments and last week guided the golf cart-sized rover off its landing platform, where it had been sending detailed images of the planet's red and dusty surface.
The final images show Spirit's robotic arm starting to investigate a football-sized rock that scientists dubbed "Adirondack." The rover is equipped with instruments that can scrape away at rocks and soil to get to a fresh layer and identify their composition.
On Wednesday, scientists said a thunderstorm near a Deep Space Network radio antenna in Canberra, Australia, disrupted controllers' efforts to initiate the drilling of the rock.
Although project managers have not yet been able to pinpoint the cause of what appears now to be technical problems, a press release issued Wednesday by the Jet Propulsion Laboratory, which operates Mars missions for NASA, said similar lapses in communication occurred during the 1997 Mars Pathfinder mission.
Spirit's twin rover, named Opportunity, is scheduled to land Saturday night in an area called the Meridiani basin on the opposite side of Mars from where Spirit is located.
The rovers are programmed to investigate the geologic history of Mars and search for clues that the planet was once a watery place that could have sustained life.