I use a Casio Privia PX-150 keyboard (very nice by the way) as a
kb controller for Cubase 7 connected via USB.
PROBLEM:
I must first turn on the KB, and then call up Cubase in order
for the two to recognize each other. When I do it in that specific
order, I’m fine. I get sound… both in and out… life is good.
HOWEVER:
If I turn off the KB, then turn it back on again, I lose the connection.
I get nothing. Just by that simple act alone, I lose my connection.
WHAT’S DA PROBLEM ?? Why can’t I just get my sound back?
I should have added: In order to get my connection back,
I actually have to CLOSE down Cubase completely, turn
the KB back on, then boot up Cubase again. Go figure??
Same thing here too. Had the problemsince C 5.
pletty of threads on this , Cubase is not usb hot swappable so if you turn your usb device off you then have to reboot Cubase .
That is the way it is 
On Windows… 
Works that way with the CC121 as well.