Is my 3D printer totally dead?
I would have posted this as a comment, since it is not a definitive answer to the question, but it was too long for a comment.

Since you were unplugging the fan when the problem started, I would suggest that it is a wiring problem.  

The first place I would look is to check the ground connection between the processor board (probably the ATMEGA Arduino), and the board that carries the stepper motor drivers.  If the ground connection is faulty, the board may still work due to sneak ground paths through signal lines.

A simple way to check is to measure the voltage from the GND pin on the ATMEGA to the ground of the driver board.  That voltage should be very close to 0.0 Volts.  You might also check the resistance (using an ohm meter) with the power turned off.  The resistance should be 0.0 ohms, or if the meter is not perfectly calibrated, it should be the same as you measure with the probes connected together.

If either of these readings isn't zero, search for a broken connection.

One sign that ground may be broken is the LCD brightness control sensitivity.  I don't have the schematic to refer to, but if the ground reference for the pot is broken, the output of the pot will be too high.

Another sign is that the stepper motors still make some sounds when they should move.

If you don't find a problem there, check through the other power and ground connections.  Be sure that all GND lines are well connected, all +5 lines are well connected, and that all +12 lines are well connected.