Check to see if the back-light is working by pressing the back-light button.
If yes, adjust the contrast setting for the LCD by holding down one of the back-light buttons and pressing the UP or DOWN arrows to make an adjustment.
Check/Change the battery. Use only Alkaline batteries.
If this does not work then you can try a ‘hard reset’. Remove one of the cells from the back of the instrument. Before refitting the last cell press and hold both the UP and DOWN arrow keys together and simultaneously press the last cell into place. If all is well the instrument should come back to life (the screen will show a horizontal line followed by a small box).
It will need to be reconnected to the PC, and “Communication” started to complete the reset.