Hey David,
Unfortunately, if the device is disconnected while updating firmware/software, it usually bricks the device, unless you would like to lean microsoldering. If you would, I would reccomend watching [https://www.youtube.com/playlist?list=PL4INaL5vWobD_CltiZXr7K46oJ33KvwBt|these videos], or [https://www.youtube.com/channel/UCPjp41qeXe1o_lp1US9TpWA|these ones]. Also, check out [guide|61535|this guide]. Sadly, I faced a similar problem with a $50,000 laser engraver. Luckily, it was only $800 dollars to replace some parts, but in your case, I don't know if it will be that easy (but much cheaper!).
Hope this helps,
-Tanner