So what do you do?
I try at all times to have a pen, a notebook, an electronic notebook and in the worse cases a SMLAVSRD - supportive mobile listening and variably successful recording device (otherwise known as a friend) on hand. (NB never attempt to use your cat as SMLAVSRD. They may appear to listen, but the playback is always flawed.)
As for white screen - white screen time is for plotting and world building. When it comes down to it free flowing writing in character or the creation of great description is huge fun, but for most of us it rarely makes a novel. Whether you're writing fantasy or keeping it contemporary you need to know where your characters live. You need to know how their locations lie in relation to each other. You need to know who is in authority, who has jurisdiction over whom. And, of course, if you're writing fantasy you need to work out if a gerb is bigger than a greeb - and that's only the start.
Plotting is more complicated. The structure of the novel has been evolving since the first psychological novel was written. - I think this was Genji, but I'm open to correction - But novels do have structure. This structure rarely just comes into being. It's a puzzle that needs piecing together. If you can get the outline of this puzzle in place then the free flow of writing the A to B parts is not only enjoyable, but tends to comes more easily. (The writing programme scrivener is hugely helpful for plot and structure development.)
And if you're feeling utterly, utterly uninspired, it's time for housekeeping ie submitting to agents, publishers and trawling through relevant blogs and news sites.
But, and this is for another entry, writers are very like fields. We also need our fallow time.