If you feel you know everything about your organization and its strengths and deficiencies, it's not the end of the world. In fact, it's the only way to fix things.
I frequently am engaged by consulting clients to solve a seemingly basic problem. With surprising regularity, a CIO or IT leader will tell me: "We don't know what we don't know." While seemingly paradoxical, acknowledging this concern is the first inkling of knowledge about a situation and the first step toward resolving a problem. When you find yourself facing the unknown and arriving at the stark realization that you don't know the size, shape, or color of the enemy you're facing, I recommend the following:
Record the symptoms
I frequently work with organizations to make their IT departments more effective. While the average CIO who calls me doesn't know what he or she doesn't know, they do have a litany of unfavorable symptoms. An IT organization may be effective, but that effectiveness comes through unending long hours and a constant feeling that IT is solving yesterday's problem. Perhaps IT is well regarded and superficially respected, but the CIO and his or her team are invited to strategic meetings after all the decisions have been made and implementation is underway. Or, as is the case with the best athletes and executives, everything may be going swimmingly, but there's a sense that there's still room for improvement. Even if you don't know the source of or solution to your concerns, acknowledging and recording observable behavior is the best place to start.
Most teams and larger organizations are reflections of their leaders. I'm not smart enough to understand the interpersonal and cultural subtleties, but an aggressive team usually is captained by a hard-driving leader, while a more sensitive and cerebral leader might have a team that makes considered maneuvers armed with all the best data.
Solicit your boss, direct reports, and trusted colleagues for their thoughts on your leadership abilities, strengths, and growth opportunities. This can be accomplished formally in the guise of tools like a 360 evaluation, or informally over a few cups of a favorite beverage. In all cases, look for trends and actionable behaviors. A singly quip that "you're a jerk" can be safely written off, but if you're repeatedly told that you set unreasonable schedules and expectations and then crucify those who fail to meet them, there's likely a problem to be investigated.
Know the business, buy the tech knowledge
Despite being the Chief Information (or Technology) Officer, it's unreasonable to assume you'll be intimately familiar with every technology trend, new device, or enterprise offering that comes down the pike. What you should be familiar with is your company and its unique processes, markets, products, and competitive landscape. For most technologies, merely expressing an interest is enough to have a parade of highly-skilled technologists lining up outside your door. If you can articulate your company's unique strengths and challenges, an occasional review with outside technical experts and vendors will clarify opportunities you didn't know existed.
Get outside insight
Similar to the above, a major cause of not knowing what you don't know is that you live it on a daily basis and lose sight of the larger picture while managing details on the ground. Just as the professional athlete may not understand some nuance of their golf swing, skiing technique, or tennis swing until detailed video and outside analysis provide illumination, getting an outsider to observe your operations and your work in it can provide an immediate spotlight. While IT spends massive sums on technical consulting, many IT leaders see true management consulting as an unnecessary expense. If you have a mind to improve but don't know where to start, this type of expertise can dramatically accelerate your path toward insight and implementation.
Not knowing what you don't know may initially be a sinking feeling, or engender concerns that you're not discharging your duties appropriately or are a fraud of some sort. This could not be further from the truth, however, as you've already identified a need for improvement and made an honest assessment that there are some gaps in your knowledge as to how to get there, or even the first steps to take. Hopefully the above suggestions will start you down the path, and frankly if you feel you know everything about your organization and its strengths and deficiencies, I'd be more worried about that than a sense that you don't know what you don't know.