Take a break is my usual plan of action, either take a walk, watch a movie, walk the dog, go to the pub or write a plan of ideas that you may be interested in in the future, this is what I like to do and it works every time.
Sometimes taking a break from what you've been thinking about too much helps you come back to it refreshed and ready to go again, there's been times when I've been trying to draw something and it just screws up every time and so I walk away and do something else like have a coffee, although I've just given up coffee and I only drink tea or hot chocolate, the five or ten minutes you take to rest your mind and your eyes away from your drawing is the best thing that you can do for your creativity.
I'm a great believer in planning for these so called non motivating days that seem to hide and wait for you to fail and by planning I mean so that you always win over them negative thoughts that do cry out and whinge that you can't draw or think of anything to draw, I'm always writing lists of ideas and sheets of notes to guide me on my way, call it my wonder sheet where I wonder what I'll draw next, well that's what I do and I take it anywhere, even on days when I'm not at my desk because that's when ideas strike.
When you're watching the telly or reading a book, ideas infiltrate your brain like planted thought suggestions, I tend to think of visual and audio helpful in bringing excellent creative ideas to the forefront of your mind and while were at it, sleep is one of the best idea generators that we can have, especially if you can remember your dreams, even if they seem vague, but interesting enough to write down moments after waking up, it's all about inspiring yourself in ways that you like and by doing the things that you like to bring about those thoughts and feelings of exhilaration.
For me, there's nothing like working solid on drawing for a whole day and creating some great drawings and pieces of art that you are proud of, so try taking a break, have a sleep if necessary or do something else that will help your creative side to shine!