Making sure you tried the import function in kicad. Can't "open" but may "import-nonkicad files" or something like that.
Also,
Open the .sch file in notepad++ and check the first few lines. May get a clue of the original program used.
Once you know the original app, may be able to find a plugin to assist or atleast know what to Google. E.g., "open altium .sch file in kicad"