If the screen is dark on startup it may be that the Contrast Ratio has been set to zero.
If the screen is bright and no text can be read it may be that the Contrast Ratio has been set to 100.
To adjust the contrast ratio perform the following steps:
1) Force the instrument off (remove the batteries if necessary)
2) Turn the instrument back on with the center button (lightning bolt).
3) Press and hold the up and down buttons to cycle the contrast setting.
4) When the display becomes legible go to Global Options/Display Settings and adjust the contrast to that level (press the center button to save).