louisiii23,
If you have a Pearl, it may be a well-known problem of memory shortage. The Pearl has very little "spare" memory. If you aren't careful, you can let your Pearl's memory fill up with messages, call logs, old calendar appointments, etc. The BlackBerry OS (whether on a Pearl or other model) is designed to free up memory when it desperately needs it by deleting old stuff in memory first, then as a last resort, deleting more important stuff. It may have deleted your calendar appointment to free up enough memory to keep the phone running. Please read these articles to help you more:
http://newbbie.com/2008/03/01/text-messages-email-call-logs-disappearing-insufficient-application-space-keep-getting-a-tumbling-hourglass/
http://newbbie.com/2008/05/03/app-review-softreset-a-silver-bullet/
Kelly