Whenever I work with controllers for sound production (midi controllers and the like) they seem to do something similiar to what you are asking about on occasion. I find that after awhile the USB ports don't carry the same voltage as they do when first plugged in at first.
Anyway when this happens to me I do two things: 1) if it has its own power source(rather than running from USB) I will pug it in and it starts to work. or 2) use a powered USB hub such as : Amazon.com: USB Highspeed 7 Port Hub w/ AC Adapter (Silver): Electronics
/\/\/\/\/\ quick amazon search, not actually the one I use