The scene depicts a father, who I believe has a shotgun slung around his back, searching for his son in what I believe is a house. He hears his son calling for him, shouting for help in a ghostly way.
He hears his son through a bathroom mirror, opening the mirror cabinet and closing it, to no avail. He also sees a nearby painting, depicting his son stuck and shouting in horror in the same bathroom mirror. A window is beside this mirror, through which he frantically reaches behind the mirror and feels nothing but the outside wall. He gets an idea and breaks the mirror, revealing another world full of darkness, except for small light emitting from a pool of water and cage with his son in it.
This location is a few stories down, and the father then climbs down with rope. My memory fails here and I only remember that he jumps into the pool with his son being chased by a humanoid monster. He swims down and then back up, where he ends up in the "real world" swimming pool. The monster then slowly comes up out of the water behind them...this is where my memory ends.
FYI, this may very well be a network movie and another clue is that I believe this movie was on cable before the year 2000.