which window balance part should I be buying

Your balance may actually still be functional, but the top sash guide is broken. These are little caps on the two top corners of the sash that keep the balance in place. When they broke, likely due to sun exposure making them brittle, the balance shoots up in the track.

This is easy enough to fix, but you do need to figure out what top sash guide you have. You can see our entire selection here. The video below will go into more detail as to what your options are.