When writers can't come up with ideas or motivation to write they are said to be suffering from writer's block. I've yet to hear of an equally descriptive term that describes what we artists are dealing with when we are stymied in a similar fashion.
Frankly it's a problem that I rarely have. My problem (if it could even be categorized as a problem) is that I quite often have too many ideas for any particular potential illustration, whether it's for a client or myself.
When I am searching for ideas and/or inspiration I do have a few tried and true methods that always seem to generate sparks of creativity. The first is to exercise. I am a runner so that means lacing up the running shoes and hitting the pavement. I have designed and illustrated many wonderful pieces of art in my mind while out for a long run.
Another great idea starter is to just flood my mind with imagery. Quite often I'll plug in some key words that encapsulate the main part of the illustration I'm conceptualizing into Google Image Search. The results of these searches never fails to get me sketching some ideas that quite often end up working very well.
I also keep a pad and pencil on my nightstand next to my bed. On occasion a great concept for an illustration comes to me right before I go to sleep.
These are the methods that work for me. Every creative person I've ever met have their own unique way of generating ideas and inspiration.