Just when they thought it was safe to go back in the water, many 401(k) investors who were regaining their comfort level with the stock market took another hit.

They certainly weren't anticipating another steep market slide so soon after losing as much as a third of their retirement savings in the financial crisis of 2008. To try and bounce back, many workers over 40 kept their money in stocks hoping to repair their losses and get ahead.

Indeed the strategy paid off for many. The more than 110 percent climb of the Standard & Poor's 500 index since the market bottomed in March 2009 restored some vigor to what had been discouraging account statements. Those who continued to contribute to their 401(k) and stayed invested, saw their balances recover and most were ahead of where they were before the market collapsed.