Unfortunatly it happened to me as well: While upgrading from the first OS to the latest (1.2.0.4), I turned off the power to early after finishing the Panel update. After powering on the screen remained black and some buttons were illuminated.
So I followed the instruction on the troubleshooting page (and some posts in this forum) sending the latest OS again via Midi and the bootloader mode. This didn't solve my issues: Now no lights are illuminated, and the screen permanently shows the PRO 3 logo. No sounds come out of the synth.
I reached out to the support, but after reading today's heartbraking news, I'm sure there are more pressing issues than this.
Hence, I was wondering, if someone here in the forum has some advice for me & maybe an older version of the OS i can try to install. It seems that some people were successful when downgrading to a prior OS, but I am unable to find them anywhere online for download.