I'm assuming you mean "stopped"? Stopped means that a stock was bought, but when it was supposed to be sold, no data for the stock ticker was no longer available. This typically happens if a company gets delisted, gets bought out or goes bankrupt. Fortunately, it is most often the former than the latter. When a stock is "stopped" a simple estimate is used for calculating the return. This estimate is 1%/month adjusted for the actual holding length for the stock in question.