Udie -
don't worry about it. Make sure you have you money diversified into several different funds.
I am 100% in stocks and have lost about $6000 so far, but I'm not worried. In fact, I'm going to INCREASE my contribution starting my next paycheck. Look at it this way, when the stock market is down, you are buy lots of shares at a reduced rate. When the stock market goes back up, you 401K will increase that much faster.
Follows the saying, "buy low, sell high". The worst thing you can do at this point (assuming you have many years to retirement, I have about 30), it to NOT put money into 401K. Now is the time to do it, so you can buy the stock when its cheap.
Course, I'm not a financial advisor and you have to use this information at your own risk

. Gotta protect myself ya know.
The stock market will come back. It has to. If it doesn't, we ALL will have a lot more to worry about then retirment at that point if you catch my drift.
Good luck