The fact that it came back on it's own is the surprise here, that means the heater itself isn't toast, and the fuses aren't blown.
This leaves 2 options
1) an intermittent hardware issue, only choice is to keep an eye on it and if it keeps happening get it in for service.
2) software glitch (not a "version 7 doesn't provide heat" glitch, but an actual issue), in this case there's the usual advice any time anything on this car doesn't work. First try a re-boot, if that fails, try it again, and if that fails call Tesla.
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he hasn't even reported cold yet... just a nice autumn day... wait until winter!