Just going to document this here - it's a confirmation of other people's findings which I've seen spread over various places.
My OP-1 has been in my bag for the last month and I've not switched it on at all. When I came to switch it on yesterday I got the dreaded "nothing happening". USB charging seemed to do nothing but I left it plugged in to a phone charger with the switch both on and off at various points. Still nothing. Switching on with the Com button held down did nothing either. What worked was:
1. UNPLUG THE USB CABLE. I don't know why.
2. Switch on with Com button down.
3. Format the disk
4. Follow instructions to upload the firmware.
Appears in my case the firmware had become corrupted (I wasn't running the beta - although I took the opportunity to upgrade to it). I am not sure if the charging helped or not.
None of the above is new in itself. Just a clear run through what worked for me.