If the display says the platform is 255C but it is cold that means the thermistor is shot.
On mine the chip was broken, presumably due to repeated expansion and contraction with temperature.
How to fix:

You will need to purchase a new surface mount thermistor with a cold temperature resistance of about 100 K ohms. Search on DigiKey for Negative Temperature Coefficient thermistors. I used two 47K in series and it worked within a couple of degrees. Cost about 18 cents.

Remove the platform. Count how many nut rotations to help getting close on re-assembly.
You can unplug the cables. Don't pull on the wires.
The thermistor on mine was a light blue color, I think labeled R29 and right near the center of the board.
Replace it.
Check with an ohm meter.
Re-assemble platform and add a couple of turns on the nuts so you don't crash the head.
Do a preheat and check temperature - a handheld infrared meter is ideal and cheap.
If the temperature is more than a few degrees off you may have to note the difference and adjust your settings to compensate.