When a company's stock is removed from an exchange, it is delisted. The reason may be good, such as the company merged with another or decided to switch exchanges. But reasons may also be bad -- not abiding by exchange rules or no longer meeting financial or size thresholds for continued listing.
Requirements vary on different exchanges, but one example often mentioned is that on the Nasdaq, share price has to remain below $1 for 30 consecutive trading days for a company to be delisted based on share price alone. Companies, however, may appeal, probably arguing that they will soon be in compliance, so the delisting process can be lengthy.