- The company has merged with another company.
- The company had a name change.
- The company has been delisted (indicated by symbols such as .PK, .OB or .OTCBB).
When a ticker symbol changes because of a merger, the company being acquired usually gives up its ticker symbol in favor of the acquiring company's symbol. Corporate actions such as mergers can often be positive for a company, especially if the company is taken over for a premium over the share price.
Sometimes, a ticker symbol changes because the company has changed its name. For example, when AOL Time Warner dropped the AOL and became simply Time Warner, it changed its symbol from AOL to TWX. A company name change generally doesn't mean much to its operations, though investors might interpret it as positive sign if it reflects a positive change in the company's overall strategy.
If a ticker symbol has had letters added to it – such as .PK, .OB or .OTCBB – this means the stock has been delisted and is no longer trading on the exchange on which you purchased it, but rather on the less liquid and more volatile over-the-counter market. More specifically, a .PK indicates that your stock is now trading on the pink sheets, while .OB or .OTCBB represents the over-the-counter bulletin board.
A stock that has been delisted is like a baseball player who has been sent from the major leagues to the minor leagues. For some reason, the stock is no longer worthy of trading on a major exchange like the New York Stock Exchange or Nasdaq, perhaps because it failed to maintain the exchange's requirements. (See also: "What are the listing requirements for the Nasdaq?")
The Nasdaq also used to change stock tickers when a company entered bankruptcy proceedings, by adding a "Q" to the end of the ticker, or when the company was delinquent with its SEC filings, by adding an "E" to the end. The Nasdaq ended this practice in January 2016, and now uses the Financial Status Indicator to denote delinquent regulatory filings or bankruptcy proceedings. (See also: "What are the fifth-letter identifiers on the Nasdaq?")