Using a pry bar or leverage to try to open a stuck window needs extreme caution. Try to break the seal of the paint or compound that is holding the window in place before applying a lot of upward pressure. Always clean the sliding mechanism immediately of any old paint, putty or other compounds before closing the window again.
Window repair is a great opportunity for you to check for potential problems with the other windows in your home. Instead of calling out a professional each time there is a problem, just call him or her out one time, getting all of your window needs taken care of.