The Spirit Mars rover who stopped responding on January 22nd on the surface of Mars ran out of RAM. That is what the scientists concluded.

Apparently, the machine runs on 128 RAM of which 32 MB is reserved for the onboard operating system and some scientific programs.
When it starts working, part of that memory gets filled with data and programs, and after a while, scientists are supposed to remotely remove part of the old data to free up memory. When that doesn't happen, the machine runs out of available memory and goes into diagnostics mode, which is exactly what happened with it on Mars.

Eventually, they were able to fix it and broadcast resumed ...