When the words get stuck, they need a little coercion and a little patience to make them come back out. Sitting down and writing when you think you have nothing to say is exactly the time when you MUST sit down and write. Write about having nothing to say (somewhat like I'm doing...), and whatever else may pop up; the asshole who cut you off in traffic this afternoon, or the ridiculous Birthers and how they need to get a life, even the speck of carpet fuzz on the floor next to your desk. Any of these are excellent ideas if you're stuck but need to do something to unstick the words from your brain. They may seem trivial or pointless, but you never know: the asshole driver could be the flawed hero of your next plotline, or the carpet fuzz could have some very interesting insights into what people carry around with them on their feet or what the room looks like from the floor.
"A little nonsense now and then is relished by the wisest men." [Gene Wilder as Willy Wonka] And don't you forget it. Write the drivel, the nonsense, the tomfoolery; write it until you're all driveled and nonsensed out. Maybe it'll help the brilliant words come forth and make themselves known in the beautiful and excellent prose you believe yourself capable of. Or maybe it'll just remain blather and you'll feel like you wasted your time writing about a piece of carpet fuzz. But, dear readers, writing is never a waste of time for people who love writing. Even when it's difficult, even when you feel like you'd rather do anything but write, there's always that blissful memory of the 3 AM frenzy when you managed to crank out 10, 20, or even 30 pages in a single night, and getting back to that feeling of accomplishment, of excitement, of ecstatic, if punch drunk, enjoyment, is the reason we writers write in the first place. Unless you're one of those writers who writes for the money. If that's the case, I'm not talking to you.
So the next time you get the dreaded W-R-I-T-E-R-S B-L-O-C-K (that might have been cuter had I been speaking rather than writing--oh well), expel the babble and incoherent gobbledygook, and maybe you'll be able to get back to the good stuff. What started out as a rambling piece of trash could turn out somewhat interesting. We can dream. Be not afraid. Write all the boring out of your brain and make way for innovation!--or at least something you can say is less sucky than the last.
A few words to take with you: hogwash, twaddle, poppycock, rot.