Raspistill also just produces a plain black image. I've tried during daylight hours so even if the IR LED which is in the box (it's a noir camera) was on the blink, the daylight would still show an image, but there's nothing at all showing.
I'm taking it from that that it's the camera which is at fault. It's a bit hard to explain how the camera is mounted, but it's basically on top of a bit of wood which has a hole cut into it, which in turn has been mounted up at the top of the box so it's looking down on the nest.
It is possible that somehow the bird has knocked the wood so that the camera has somehow come loose and is no longer pointing directly through the hole but I would still expect it to pick up some light and give a picture of sorts but this is completely blank. The streaming software is showing 25fps so I presume it is actually sending some data out, albeit nothing worth looking at!
I guess it's a case of just waiting until the birds have flown and investigating further then unfortunately