Big investment banks, like Goldman Sachs, Merrill Lynch and Lehman Brothers, have been telling holders of so-called auction-rate securities that they cannot withdraw the funds. This affects people, mainly companies and wealthy people who buy them from issuing municipalities and public authorities. They are pitched as safe investments. And some investors are outraged. The New York Times explains that banks hold auctions that set rates and give holders the ability to exit the investment. But these auctions have been "failing," in that there are not enough investors to participate. So the investments are essentially frozen. The rates reset higher so it may not be a total loss if the market eventually starts working again.  

For more:
- here's the New York Times article