Art classes. Not only will they help you improve your drawing ability, but they'll most likely force you to draw things you've never drawn before. Trying different things helps you develop a new vision of life, which in turns gives you inspiration, which in turns makes you wanna try drawing! Simple as that! If you can't afford classes, drag a sketchbook along and force yourself to draw from nature in unusual places, like the bus of in piublic spaces.
I haven't been out of it so long as couple weeks recently but I know the feeling. I wish I could say something good as advice but I honestly don't know why I can't draw some days or others I can't do anything BUT draw.