Anet A8 reading 739°C from the extruder thermistor! I accidentally let the print head of my Anet A8 crash into one of 2 clips that are holding a glass sheet onto the hot bed, and suddenly the screen went blank and the printer rebooted, then it started reading 739°C from the extruder thermistor. I tried changing the thermistor, but that didn't have any effect. Also tried swapping the extruder thermistor with the hot bed thermistor, but there was no effect on both. I checked both the connector and the socket of the thermistor for shorts but found none. Any ideas about the reason this is happening?

-Using Skynet3D 2.3.2 on the stock Anet board.

EDIT

I tried burning the bootloader and flashing the firmware onto the Anet V1.0 board using an Arduino UNO as an ISP, but that had absolutely no effect.

EDIT 2

I measured R41 located next to the hotend thermistor header

and found out it has a resistance of 1.5kΩ, while it should have a resistance of 4.7kΩ, so I suspect this is the main reason behind this high reading. Now the only thing left to figure out is how the resistance of this resistor changed.

I was able to figure out which resistor to measure with the help of this schematic: https://github.com/ralf-e/ANET-3D-Board-V1.0/blob/master/ANET3D_Board_Schematic.png

EDIT 3
I tested T56 (located near the headers) and T55 (located near the ATMEGA1284P) for continuity, and found out there's no connection between those, while they should be connected according to the schematics. I also checked the hotbed's terminals T54 and T53 and found continuity between them, which  means the problem might be in the trace between the thermistor header and input pin of the ATMEGA chip (this trace is VERY thin, so any overcurrent might cut it), or any component in this trace.
I had the same issue. Hot-end temperature reading stuck at 209 degrees even with hot-end thermistor disconnected  or swapped with bed thermistor on the Anet A8 mainboard.

After ordering and swapping the AtMega1284p (using jtagice3 and hot-air soldering station) and the 4.7 Kohm resistor (which measured 2.06Kohm on the board and 4.7kohm off the board) to no avail, the only component left in the circuit that could cause the failure was the capacitor.

Swapping the capacitor C47 fixed the stuck reading for me. I used a 10uF instead of a 15uF capacitor as indicated in the schematic. I do not believe it makes much of a difference, and they are much more common. Make sure the capacitor can withstand at least 10V, but I got a 30V one, because the price was similar.

To any body that runs into this issue, before spending time and money swapping components like I did, measure the voltage of the node between R41 and C47 with respect to ground. This is labeled T56 or test point 56. Compare this voltage to T54, T60, T58, and T62 where similar circuits are placed. You should get a voltage of around 5V ( the pull-up voltage) in the functioning circuits, since we are talking about DC voltage, the capacitor should charge up nearly instantly when the the board is powered, and behave as an open-circuit.

In my case, C47 was almost in full short-circuit, and I read a low voltage on T56 ( ~0.5v). 

Since the capacitor was in a low impedance failure mode, the 5V supply voltage of the voltage divider circuit was not enough to power it, hence the fixed temperature readings of the hot-end thermistor ADC channel regardless of the thermistor being connected or not.

Good luck!