Just thinking... If expression pedal loses its calibration for some reason, does it go 100% or 0?
Would come in handy if we know what is the last thing you did with HD before that startup when you noticed it didn't work. Anyways. I can't know, what you've already tested, so i may be just heating the house by chatting here.
I'd start by pulling every plug out. Then Power cable in. After that the guitar. Amp cable of headphones would be helpful also.
Move them switches next to expression pedal few times (to make sure they are not left in between. Not sure if they can but just in case)
Step on expression pedal so it switches from 2 -->1 (or the other way around)
Recalibrate expression pedal (just in case)
Connect to a puter and re-install everything to it (if i remember it corrctly, needs pedal calibraton after that)
Yell: "MEDIIIIC!" (carry it to a repairsman)