The violation of federal and state securities investing and trading laws is known as securities fraud. This can include a stockbroker who gives bad investment advice, or who doesn't explain the risks of his recommended investments, or who makes unauthorized or unnecessary trades on your account. It can also include companies that make false statements or withhold information about their prospects. One securities fraud example was the infamous Enron scandal, which allegedly involved top executives manipulating financial reports and filing false tax reports.
Even after you've lost money, you have rights if you believe you are a victim of securities fraud. In some cases you may wish to consult an attorney and consider suing a publicly traded company, while stockbroker fraud may be handled through a binding arbitration (dispute-resolution) process.