It's only a loss if you pull your money out of the stock market and/or mutual funds. You lock in the loss that you're seeing on paper. At some point in the future, the stock market will eventually go up. Short-term money, okay, you may want to consider locking in the loss and putting it into a CD or FDIC-insured money-market, but if you're in it for the long haul like yours truly, make no sudden moves.* What goes down will eventually go up and vice-versa. It's the way of things.
Follow advice at your own risk; I'm not a financial planner