I had a major panic yesterday. I've been through hell these last two years (personal reasons) and have very much neglected my hobby of bad music making. So, yesterday I decided I'd upgrade to the latest OS 126.96.36.199 (I think!!). I have a MacbookPro with 10.9.2 and I use Logic Pro 9.
So, I downloaded the latest OS from the Access www site and started the upgrade process. About 2 minutes in, my TI (Keys version), just simply turned itself off - and at first I thought this might be part of the upgrade process, however, after ten mins, it remained off and I knew there was something wrong.
I then spotted on the upgrade app running on the Mac, that there were instructions to remove cables etc, and press the Exit button at the same time as re-inserting the power cable, then re-run the upgrade process again. i did this, re-established all cable connections and sure enough, everything resumed and ended up well.
I will admit to being a tad panicky, especially when the TI wouldn't power on, but it didn't take long for everything to resume after following the instructions.
I wanted to post this in case anyone else out there sees similar power downs when they upgrade. I am not sure if I was simply lucky, but it all worked in the end.