<p>My OctPrint and Monoprice Maker Select IIIP (A Wanhao i3 Duplicator Plus clone) were working fine, but suddenly today the hot end won't heat up anymore. I tried disconnecting the OctoPrint USB and resetting the printer power, and it still couldn't heat up the extruder, such as through the filament menu.</p>
<p>But then it got weird.</p>
<p>I left it off for a few hours and turned it on to see if it was getting 12 V to the hot end, and it was heating up again!  So I plugged in the OctopPrint and we're back to square one, the extruder has just been cooling down. I know the thermistor is working, because it's accurately following the temp, such as following the cooldown after the heating stopped working again.</p>
<p>Is it possibly I have a dead hot end and for some reason it temporarily started to work again?  Maybe an intermittent short?</p>
<p>I guess the next step is to open up the base and look at the connector to the motherboard, and or measure for 12 V</p>

This has nothing to do with OctoPrint itself, the cause is related to the printer itself, not the print server running the printer.
This is a pretty commonly seen issue (usually seen at heated beds), this is caused by faulty wires/cables or connectors. This usually happens after a vast period of usage. You should (periodically) check the cables and connectors. You could even test if the heater cartridge works by connecting it directly to the power supply.
Considering the limited amount of costs involved to replace the heater cartridge, it is preferred to replace the heater with a similar specification heater element (voltage and power).