Here's the answer for me (I inherited a set of the he4's). I removed the plug (didn't want to get zapped) then removed the top, exposing the board that the electronic ribbons are plugged into. I removed and reseated these ribbons (if I recall correctly there were only two). Plugged back in to test. Pushed buttons to see if display would light up...and voila!
I realize that the problem could be completely different from what my machine was experiencing... but for mine it was a connection issue with how the ribbons are seated. There's a way to improve the contact on those connections with a contact cleaner spray/lubricant. I haven't messed with any yet, but will do the next go around when it needs another fix.