You may want to consider reviewing Boglehead wiki link I posted, specially the section about Asset Allocation. If all your investment is in S&P500, you are exposed to US Large Company stocks only. You may want to consider diversifying your portfolio to other asset classes. At minimum, you want to setup 3 ETF/fund portfolio: Total stock market, Investment Bond, and International Equity.

Studies have shown that a 60:40 stock:bond portfolio performs almost as well as all stock portfolio but with lower yearly volatility.