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I was in the middle of a 20 hour print and at hour 17 it just stopped and the temperature will read as "def", and when I turn off and on again it reads normal then goes up to "def". What would cause this and why would it start like that randomly? Assuming something with thermistor, but no idea why.

Thermistor is the reason, and you need to check your connections. Most likely culprit is the solder connection at the hotend thermistor itself.