The expression is absolutely true that money lost in the stock market isn't really lost until you take out your investment. Until then, just think of it as a 'clerical error' that the bank is working to correct for you. If you are far away from retirement then there is really nothing to worry about at all. All things being equal, history shows that your investments will return to their 'normal' levels before you get the chance to walk away from the workplace forever. If you are close to retirement, however (or are, in fact retired already), then there are some things you may want to consider for your investment to keep your retirement secure:
If you were planning on retiring within a few years and you've seen the value of your retirement investments drop significantly in the last several months, now is not the time to panic and pull your "losses" out of the market, making loss on paper a reality. There is very likely to be a significant return on the markets when they stabilize and you will probably get a lot, if not all of your investment back. Also consider that retirement is not a lump-sum deal where you need all of the thousands or millions you have saved at one time - it's not like buying a one-time ticket somewhere. You just need a certain amount to live on today, and tomorrow and next month and next year. If you've been investing wisely enough to have an amount for full retirement, you should still have enough in your investments to last you a year or two (or more). Continue your plans for retirement, though wisdom would dictate that you might want to cut back a little on planned splurges like vacations around the world, etc. until the market again becomes more stable and you start seeing a return of the investment you made.
If others around you are worried and talking about pulling their money out of the market before it gets worse - reassure them and advise against it, there is very much a 'domino effect' going on in the markets around the world right now. The more people get worried and stop investing, the lower investments will sink, the more people show confidence and keep investing, the better the markets will respond and rebound quickly. Decide that you will be the strong anchor in the storm and those around you will feed your confidence into their own little corner of the market and help the whole.