Rafael - If you cannot find your old 401k plan on the Brightscope website, feel free to check out the US Department of Labor website. On their site, you can also find contact information to inquiry with them what you can do in regards to your old plan.
If you know your old employer, you should be able to go to the home page, http://www.brightscope.com, and in the search box on the right hand side, type in your employer and find out the 401(k) information. Just to confirm that this approach works, I looked up Circuit City, which has been out of business for a few years, and it was there, so you should be able to find your old employer. Once you find the 401(k) plan administrator, contact them directly.
All you need is an old statement. Call the custodian, or the company that sent you the statement. They can provide you with a current balance and send you a new statement. It is possible that you have changed your address, and they need to update it.
Consider rolling the balance of your money into an IRA. If you rollover your 401(k) to a ‘rollover’ IRA, you will likely have more freedom in selecting investment choices that may be better suited to your personal investment style. You could also have more control over fees.
Consult a local qualified investment advisor. Find one that you feel comfortable with. Ask questions.