Why am I not getting any levels? Why am I not getting any sound? What have I changed?
Oh, wait. I haven't changed anything.
And then I managed to fat finger the Cubase remote so that I didn't notice that it wasn't actually recording until I went into the engineering booth and saw that there were no tracks.
Well, it's good to have something go wrong that is:
a) Clearly my fault
b) Easy to fix. :)