We've all had the experience of needing to create something and having absolutely no idea where to start. It is often known as “writer's block” or “blank-page syndrome” because of its notoriety with authors, but it happens to all creative types. It may come from the feeling of being overwhelmed by the task at hand or from distractions that preclude focusing clearly. Sometimes there's no identifiable reason; it's just that nothing comes to mind.
There are many ways people have found to deal with writer's block. Procrastinating until the situation is critical is a popular tactic, especially among those that do their best work under pressure. But sometimes pressure can have the opposite effect, causing the block or exacerbating it.
Another technique some use to break through is to start doing something (rather than sitting paralyzed), with the idea that the inspiration will come as a result of engaging in the process of generation. That method has worked for me when I had a general idea and was searching for a definite one, but the sensation of being entirely clueless was more daunting. When I'm blocked, as opposed to being open but with nothing specific to work from, it doesn't help me to sit down and start puttering around.
One of two techniques usually works for me: to either slug it out or back off. In a sense, both tactics amount to taking one or more steps back from the problem to take some heat off of my creative mechanism.
Slugging it out means taking time to think about what I'm trying to accomplish. That shifts the focus from content to context. If I need to compose a piece of music and have nowhere from which to start, I think about how it will be used and what the priorities are for that situation. If an idea doesn't come to me, then I'll try to jump-start the idea machine by thinking about another piece of music that totally epitomizes what I'm attempting to do, and pondering why it is so successful. In other words, I turn from creative right brain to analytical left brain — not to compose, but to examine the framework. That lets the right brain get out of the hot seat and into the peanut gallery. With pressure removed, an idea will usually pop up at some point in my analysis.
My other method is to let go of it altogether for the moment and do something else. I am a big believer in the brain's ability to do background processing on a problem, and I frequently find that a new idea or the solution to a problem comes to me when I'm doing something entirely unrelated. It may seem contradictory, especially when under deadline, to put off cranking out the product. It can prove faster, however, to let it go than to keep pounding my head against the wall. Too many unsuccessful direct assaults on the problem can compound the blockage with frustration. Leaving it alone, like slugging it out, takes the pressure off and lets my mind work on the problem in its own mysterious way. The key to that is trusting that there is enough interest in or urgency to the situation to drive the “back burner” process, rather than forgetting the whole affair. If “out of mind” means fully “out of mind,” this tactic will fail.
For music composition, specifically, I can often use a third method: listen to music, especially live music. Hearing a good live performer frequently seems to start music flowing in my head. On more than one occasion, I have furiously scribbled notes during a performance.
Although substances have been known to sometimes foster inspiration, I can't recommend obliterating oneself every time the creative process is stalled! Even if creative blockage is rare, you'll get yourself in trouble with that method.
Finding effective methods for breaking through blank-page syndrome is an individual process. The amusing irony is that the best avenue to determining those methods is to be creative and experiment with different ideas. I hope you don't come up empty on that one.