All I can remember seeing is a standard, upright piano, which seemed to be possessed or sentient, pushing someone off a stage. A young girl plays a single key on this piano later on and it seems to ‘respond’ to her. I’ve found a description of it somewhere on the internet, but for some reason no title.
The Demons of Ludlow (1983)?
http://www.imdb.com/title/tt0223128/?ref_=fn_al_tt_1