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.
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.
The main thing to remember in this section is not to provide new data, but to explain in detail data that has already been provided and to provide the support for that data.
Do not try to write your business plan in one sitting. As I mentioned in the beginning, for a traditional brick and mortar business, it could take in excess of 100 hours to compile all of the information needed into a comprehensive but yet understandable document. For online businesses, probably not that long. But your final product should be well thought out, well documented and easily understandable.