I bought $100 worth of PI tokens two months ago, but the exchange I bought it from is no longer working. How can I sell it?
If it's on that exchange then it's gone forever together with them. If they are a legitimate exchange, they should left you an email on how to move your funds before they've been gone. But if they've just been gone out of nowhere, kavej is right that you've been likely scammed for that amount.
Recently, I saw many cases which people claiming about losing access to exchanges claiming to list PI network, or their balances in PI got frozen for undetermined period. Therefore, early users who succeeded to withdraw from PI main wallet got the chance to sell their points (it's not a crypto token by all means), while other users who thought to store the points in exchanges or left them in the main wallet found that their balances frozen for years. A friend of main gets the notification that their balances will be unlocked by 2028. Those are all indications that the project is a ponzi scam scheme.