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.
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.