Check that the lock is not engaged.

An extrusion may have broke and the piece has fallen into the edge
space (alongside the sash's edge), if there is space.

Check out this link and run through the pictures, also:
http://www.thewindowconnection.net/c.../suppliers.htm Maybe you
can email this company/link and ask how to remove the sash.

I'm not sure how to remove one of these sashes. I would have to see/
examine it.

Sonny