Currently, there are 11 codes used by NASDAQ and other exchanges to specify why trading has been halted for the specific security. The OTCBB currently uses 5 codes.  T-1 and T-12 halt codes are the most commonly used. 

What is a T-1 Stock Halt?  A T-1 trading halt is when NASDAQ has halted trading in a stock due to the fact that "material" news will be released soon. A T-1 code is used when News is Pending 

What is a T-2 Stock Halt? Once the news has been released, the trading halt code will change to T.2.  This means the dissemination process has begun through a Regulation FD compliant method, and the stock is still halted, but that the news has been released. 

What is a T-3 Stock Halt? A T-3 halt code is eventually used when the news has been fully disseminated and there has now been a time specified as to when the stock will re-open for trading. This is called Halt Resumption Times and NASDAQ will display 2 specific times: 

1.  A time when market participants can enter quotations 

2.  A time at which the security will be released for trading. 

What is a T-6 Stock Halt?  A T-6 halt code means Extraordinary Market Activity. If NASDAQ believes that such extraordinary market activity is caused by the misuse or malfunction of an electronic quotation, communication, reporting or execution system, a T-6 code is used 

What is a T-8 Stock Halt?  A T-8 halt code is used for ETF's that are misfunctioning

What is a T-12 Stock Halt? A T-12 halt code is used when NASDAQ or the listing market requests additional information 

What is an H-4 Stock Halt? An H-4 code is used when trading is halted due to the company's non-compliance with NASDAQ listing requirements. 

What is an H-9 Stock Halt? When a company is not current in its required filings, trading is halted and a H-9 code is used.   

What is an H-10 Stock Halt?  An H-10 halt occurs when The Securities and Exchange Commission (SEC) has suspended trading in this stock. 

What is an H-11 Stock Halt? An H-11 code refers to Regulatory Concern. Trading is halted in conjunction with another exchange or market for regulatory reasons. 

What is a D stock Halt? A D Stock halt code occurs when a security has been deleted from NASDAQ/CQS 

There is nothing an investor can do when a stock is halted. Shares can neither be bought nor sold, so there is no need to contact a broker. When the stock does re-open for trading though, many people will be frantically buying or selling, depending on the news, so be advised not to panic, and wait at least 30 minutes after the halt before placing the order.  Hope for the best (buyout or major piece of positive news), but prepare for the worst (missed earnings, restatement of earnings, etc).