If you've got shares in a company that's filed for U.S. Chapter 11 bankruptcy, that sucks, it really does. I've been there before and you may lose your entire investment. If there's still a market for your shares and you can sell them, you may want to just accept the loss and get out with what you can.

However, shares of bankrupt companies are often delisted once bankrupt, since the company no longer meets minimum exchange listing requirements. If you're stuck holding shares with no market, you could lose everything – but that's not always the case:

Chapter 11 isn't total and final bankruptcy where the company ceases to exist after liquidation of its assets to pay off its debts. Rather, Chapter 11 is a section of the U.S. Bankruptcy Code that permits a company to attempt to reorganize (or renegotiate) its debt obligations.

During Chapter 11 reorganization, a company can negotiate with its creditors for a better arrangement. They typically need to demonstrate to creditors that without the burden of the heavy debt, they could achieve profitability. Such reorganization often involves creditors taking complete or majority ownership of the company when it emerges from Chapter 11 through a debt-for-equity swap.

That's why you, as an investor before the bankruptcy, are very likely to get nothing or just pennies on the dollar. Any equity you may be left holding will be considerably diluted in value. It's rare that shareholders before a Chapter 11 bankruptcy still retain any equity after the company emerges from Chapter 11, but it is possible.

But it varies from bankruptcy to bankruptcy and it can be complex as montyloree pointed out.

Investopedia has a great article: An Overview of Corporate Bankruptcy. Here's an excerpt:

If a company you've got a stake in files for bankruptcy, chances are you'll get back pennies to the dollar. Different bankruptcy proceedings or filings generally give some idea as to whether the average investor will get back all or a portion of his investment, but even that is determined on a case-by-case basis. There is also a pecking order of creditors and investors of who get paid back first, second and last. In this article, we'll explain what happens when a public company files for protection under U.S. bankruptcy laws and how it affects investors. [...]

How It Affects Investors [...]

When your company goes bankrupt, there is a very good chance you will not get back the full value of your investment. In fact, there is a chance you won't get anything back. [...]

Wikipedia has a good article on Chapter 11 bankruptcy at Chapter 11, Title 11, United States Code.