Assuming you still work at the company and you're not 59.5 years old, you would have to pay income taxes plus a 10% penalty for withdrawing the funds. If are are 59.5 years of age, the 10% penalty goes away.
In this case, you're probably better off to change your investment mix if you're concerned about a further decline in value. You should be able to invest in CDs or some other lower risk option.
The 401-k is not protected from investment risk, if you have investments that have risk.
If the variables I described in the first paragraph don't apply, shit changes.