It depends on when you invested and how old you are.
You buy now and you do not plan on retiring for 30 years from on and you can make some great returns.
If you invested last year and you plan on retiring in 2 years you are screwed.
The overall jist from economists and financial planners is if you do not need the money in 5 years, leave it where it is - however, no one can completely predict the stock market.
If you look historically though, (and I've studied business cycles) you will see that the market has gone up and down over the years, but overall the market has consistently increased over time. I wish I could graph it here for you, because it is more difficult to explain than see. Basicially consider a graph you start at the bottom and then 20 years from now the point is much higher, but in between you see the lines moving up and down with dips and hills.