MIDI controller and Ableton Problem
I've encountered problems where running in power-savings mode or even with just the power unplugged, the machine will power down USB devices to save power.
Were you plugged in?
This can be an insufficient power problem. Make sure you are using the correct adaptor and that computer is getting enough Power.
MIDI can be CRAZY sometimes, so when I run into problems like that - a simple reboot and unplug / replug all the cables usually fixes the problem.