Most of the hardware is controlled by software...
so if the software had a bug or glitch, the hardware can't work properly. Actually, the reverse is true as well - hardware glitches can cause software problems. I'm not saying that's the cause of Spirit's problem, just answering your question in general terms. If that's determined to be the case, I still hold out hope for an eventual recovery. The wizards at JPL have a long track record of fixing software glitches in short order. Those folks are GOOD.