What happens when a stock I hold stops trading?

Maintenance Margin is the amount of funds which must be maintained in order to continue holding a position. In the U.S., the rules of the listing exchanges specify the maintenance margin requirements on security transactions subject to SEC approval. The exchange minimum maintenance margin requirement for long stock positions is currently set at 25% although brokers can add any amount of a 'house margin' requirement in excess of that, particularly where the security is considered low-priced or subject to volatile price changes. The exchange minimum maintenance margin requirement for short stock positions is currently set at 30%. You can monitor your margin values in real time using your TWS/Mosaic Account Window. …

Accounts owned by the same beneficial owner are considered consolidated when determining the ability to sell a stock short. If account A is long 100 shares and account B is short 100 shares, a new order to sell shares in either account A or account B will be considered a short sale, as overall the accounts hold a 0 position. The new sell order would be subject to any applicable locate requirements…

A delisted short stock position will remain in your account until either the stock starts trading again or until the time the DTC removes the shares from your account. It is unknown how long it will take for the company to dissolve and for the DTC to remove the shares…

The minimum statutory margin requirement for long US stock in a Reg T margin account during the trading hours from 09:30 EST thru 16:00 EST is set to no less than 25% by Finra. At 15:50 EST the US Federal Reserve’s Reg T margin of 50% goes into effect. A Broker can set a margin requirement to any amount higher than the statutory minimum. For example, IBKR has a house margin that can be up to 500% for a short stock position. …