Coincidentally, I had this yesterday doing the same thing.
I reported it to Squap, and they believe they have fixed it.
good news… I found and easy fix/workaround for the time being.
- remove the sdcard, and you will find a file called something like CAL.CAL (its not that but its similar)
this is the calibration file, and its not been stored properly - you will find it is zero bytes.
- delete it!
- now you can plug hermod back in, and it will boot
- now redo the calibration process.
I’ve redone the calibration a few times and this only happened once to me, so its not going to happen every time you recalibrate… and as I say, Squarp are aware and have a fix in the pipeline
(you could also let squarp know via the contact form on their website)