I am late to this thread (sorry), and I don't have a Nova but I have run into this issue in the past with both my Bernina 910 and 930 when I have not pressed the hook/race cover back down firmly enough after cleaning/lubing the hook/race... there is a little catch on the left and it is very springy and I find I really have to press it back hard to make it 'click' soundly. Sometimes I have pressed it back, though it was ok, and then I have had serious thread tangle around the bobbin case and hook, because the hook has slipped out of position ever so slightly... although the machine makes a stitch, the thread is not completely 'released' from the hook, but instead tangles around the hook/bobbin mechanism, and then the machine makes another stitch, and so on, until after 3-4 stitches you get a thread jam as the tangle intensifies underneath and the needle will no longer move, and instead jams solidly in your fabric.
Then I have had to first unscrew the needle and let it hang loose, release the bobbin cover plate and snip carefully at the threads I can see in order to begin to release the stuck bobbin mechanism. After you have snipped enough thread you can then lift the needle up and out. With more snipping of remaining tangled threads the bobbin should be freed too. Then I get the tweezers out and make sure I've removed all of the bits of snipped thread.
Many moons ago it happened with my 807 and I knocked the timing out completely trying to yank the bobbin case out, thread tangles and all
, so these days if it happens I carefully snip and try to be gentle. Plus I'd check the hook 'point' is sharp too, and not flattened or twisted, as can happen if they are dropped.
My Bernina 807 used to make thread nests on the underside at the start of seams when I had put the bobbin into the bobbin case with the thread running anticlockwise
, rather than clockwise as it ought to be (as you look down on the bobbin in the case).
Best of luck with sorting it out