If you mean you have money that you wish to put into a certificate of deposit, yes you can do that.
If you mean that you have stocks, and that you wish to sell them and place the resulting money into a certificate of deposit, yes you can do that as well.
However, keep in mind that long term investors see a drop in the market as a great time to hunt for bargains. Also if you were buying funds and the money allocation you set aside remains the same, you will end up buying even more shares for a fund that has the potential to rebound if it is a solid long term investment.
You might also wish to consider that the strategy of buying high and selling low is not the best way to go.
Best of luck.