Spring? On side of window?

Thanks for posting. Hopefully I can help. That metal channel is called a channel balance. This is a spring-loaded mechanism on both sides of the window sash, that regulates the sashes movement. These balances are held in place by Top Sash Guides. When these nylon pieces break, the balance shoots past the sash, as what has happened to you. See the videos on this page that explain how to remove and replace broken Top Sash Guides. You will need to remove an undamaged guide and compare it to our selection here.