So, although a detailed business plan may not be required for an online business, I am going to include it here so you can at least look at and consider each section and determine yourself if it applies to your business.
If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.
Another key element to the marketing section of your business plan is an outline of your marketing objectives, strategies, and tactics. Writing down the avenues you travel in order to market your business will afford you the opportunity to record what worked and what didn't work. You must be able to measure and calculate the results of your marketing efforts, otherwise, what's the point? If you don't know if something is working for or against you, then it's working against you.
It may not be pleasant to imagine all the "what ifs," but doing it now and planning for those unexpected events will improve your company's chances of surviving a storm. For an excellent step_by_step guide on the details of developing a risk management plan, see the article "How to Develop a Risk Management Plan," by Charles Tremper at wikiHow.com.