Despite your best efforts, sometimes a business just doesn't take off the way you expected. The unfortunate fact is that a large percentage of new small businesses fail. But then, most small business owners don't bother to create a written business plan unless they are absolutely required to (as is usually the case if you need outside investors or bank financing). Without the benefits that a written plan can provide, it's just that much harder to cope when your business isn't meeting your goals.
You, however, have a written plan, so let's see how it can help you out when things go wrong. The starting point will be to review the contingency plans you created at the time you drafted the business plan. Your contingency plans identify and evaluate those factors likely to have an impact on your business. Unless something new and unexpected has arisen, it's very likely that you already have at least potentially identified the problem. Just as you used the planning process as a way to organize and develop your business plan, you can now use the plan as the foundation for a process to assess the adverse results and to determine how best to respond.