I have a few vinyl windows that don't stay up. They are all along the same wall, so I don't know if that means the house is shifting, or it's just a coincidence.

I opened the bottom sash by releasing the springs at the top and then re-closed it - hoping that would help. It did not.

At the bottom of the sash, it is attached to a plastic piece on both the left and right that runs inside a track. The plastic pieces are not attached to a string. Other windows in the house have a string that connects to the plastic pieces and run along the side of the window. I don't know how to reattach the string to the window.

Any suggestions?

I figured it out. But can't figure out how to delete.. .
double hung vinyl window spring mechanism repair - DoItYourself.com Community Forums (http://forum.doityourself.com/doors-windows/86003-double-hung-vinyl-window-spring-mechanism-repair.html)

I used a screw driver to unlock and pull up the plastic track and then pull down the string. Now, it works.

No need to delete. Others may find your post and solution helpful.