I invested in a start-up via a convertible note. The business has ceased operations and I am curious how to write off this investment.

You will report as the sale of an investment the same way you would report the sale of a stock.    While working in your return:

• In the income section, Select Investments and Savings....
• On the page "Did you have investment income in 2024?"  Answer Yes
• On the page "Let's Import your tax info" click on "Enter a different way"
• Select "Stocks, Bonds, Mutual Funds" on the OK let's start with one investment type
• When asked "Which bank or brokerage is on our 1099-B?" Give a description of the investment for the bank or brokerage, leave the other fields blank. Click Continue
• Answer No to the next three questions
• Answer Yes to "Did you buy every investment listed...."
• Select "One by One"
• Continue past the "1099-B"
• On the page "Now, enter one sale..." for Sales section, select either "Short-term did not receive 1099-B form" or "Long-term did not receive 1099-B form"
• For type of investment select Bond & give a brief description
• Enter the date acquired and the date you learned they were no longer in business
• Your proceeds will be $0.  You cost basis will be what you paid for the Note
• On the next page, click Continue, and your done