Korg Volca Bass stopped working - help needed!

I have a Korg Volca Bass that has stopped working after being in storage for over a year. When I power it on with batteries and/or with the 9V power supply, it’s now stuck on the initial screen where “Volca Bass” scrolls across the LED screen and all the red LED’s below the 16 bottom sequencer keys are lit except for key 16, which is flashing. A few other random LED’s flash on & off but that’s it - it’s stuck on the initial state “perma-scroll”, even if it’s left on for an hour.

Does anyone have any suggestions? I bought this second-hand and it’s way out of warranty, so any repairs are up to me. I can troubleshoot and solder, but the massive amount of SMD components is a bit daunting to say the least! I’ve done the system reset (power on holding MEMORY+FUNC keys, then hitting the record button) but that does nothing.

Thanks for any help.