When that dull boring lethargic feeling happens in your mind, you have one option.... Recharge your thought processes.
Its easy to say I shall wait for inspiration, but as we all know inspiration does not get delivered in your letter box nor does exist on a shelf in a shop.
How to recharge??? Just do it no matter how small in purpose. It will grow into a habit of bigger "doing its" and lead to your recharge of purpose.