Just make some marks on the paper and express yourself, do it for as long as you
can, and when you
simply cant focus anymore, go and get
a drink of water, or look at the stars/clouds, then go back to scribbling. If you find you want to draw something in particular, go for it, but dont get cought up into thinking "I cant draw im no good at drawing". Try some paint or colored pencils!
Also I know its a broad concept, but do you excercise during the day?