What to do when your African violet gets a long stem or neck
Scrape the neck with a sharp blade or your fingernail to expose the tissue underneath. Dust the neck lightly with rooting hormone. You must now 'bury' the neck in the soil. You will probably have to cut off some (or even all) of the rootball, but don't worry. The buried and scraped portion of the neck will produce new root.
Your plant now needs to spend all of its energy on new roots so remove any blossoms. To prevent the plant from wilting or going into shock from losing part of its root system, place the plant under a dome or inside a Ziploc bag to create added humidity. This acts like an ICU.