The USS Arizona Memorial might be closed for various reasons, including repairs and maintenance. It may also be closed if the weather is too severe to safely operate the shuttle boats that take visitors to the memorial.
There is plenty to see and do around Pearl Harbor, even if the USS Arizona Memorial is closed. Taking a narrated harbor tour (without actually boarding), along with visiting the Pearl Harbor National Memorial and Visitor Center, might be the next best thing.
For a full list of things to do near Pearl Harbor, check out this article.