nasa press release:

Ground controllers were able to send commands to the Mars Exploration Rover Spirit early Wednesday and received a simple signal acknowledging that the rover heard them, but they did not receive expected scientific and engineering data during scheduled communication passes during the rest of that martian day.

Project managers have not yet determined the cause, but similar events occurred several times during the Mars Pathfinder mission. The team is examining a number of different scenarios, some of which would be resolved when the rover wakes up after powering down at the end of the martian day (around midday Pacific time Wednesday).