We had a heat wave predicted for today. It didn't actually get that hot - except inside acme.com. I got home at 9pm and a motherboard alert tone was going. Opened up the case and the problem was obvious - a wire had gotten caught in the CPU fan and stopped it. I moved the wire away, the fan started, and the alert tone stopped.
Here's the CPU temperature graph. Looks like it didn't get hot enough to fry.
As for why the wire moved, I found a rotted rubber band at the bottom of the case - time and temperature did it in. Tie-wraps are much better.