Craigslist ads expire within 7-45 days depending on the ad type and then are no longer viewable on craigslist.org. Creating a Craigslist account is recommended if you want to view up to the past 2 years of your old ads. However, Craigslist does not have an ad archive you can access.
If that doesn’t work, it gets difficult to find expired ad postings. You can submit a subpoena if there is a legal reason you need to see the post. Otherwise, you have to hope an online archive saved a record of that ad URL from craigslist.org.