Think of your retirement fund (401(k), IRA) as a company you are priming and prepping to give you a paycheck for your later years. You start with a small investment in the business and slowly buy equipment and marketing as you get paid. As you are growing the business, you are keeping your focus on the day you retire and what the business will look like then. You grow it to produce the most monthly income you possibly can and make smart investments for growth when opportunities arise. You invest in things that have good long-term potential and that fit in with your timeline of being the most profitable when you retire. Well, the time to make a good investment decision is now, only now you are investing in what you think of as "cash", not a business.
Your retirement fund is not "cash", it is an investment just like the business example above. Sometimes you invest in something and have to wait to see the payoff. We are seeing lows in the market we may not see again; wouldn't it be a prudent long-term investment to buy into the market now? If you are contributing to a 401(k), you are buying low and you should hope it stays low for years. Why not? That way you buy low for years and sell when you retire. This is where seeing opportunities when others see gloom pays off.
If you keep a level head about you, know that the facts always ring true and that much of the news going around is just that, you should be able to stay invested for the long-haul and learn a lot along the way. That is exactly what all of this is, a learning lesson.