DTC Freezes
Occasionally a problem may arise with a company or its securities on deposit at The Depository Trust Company (DTC). In some of those cases DTC may impose a “chill” or a “freeze” on all the company’s securities. A “freeze” is a discontinuation of all services at DTC. Freezes may last a few days or an extended period of time, depending on the reason for the freeze. If the reasons for the freeze cannot be rectified, then the security will generally be removed from DTC, and securities transactions in that security will no longer be eligible to be cleared at any registered clearing agency. For more information on DTC chills and freezes, please read our investor bulletin “DTC Chills and Freezes.”
http://www.sec.gov/answers/dtc-freezes.htm