If you used fusible interfacing, it would be stuck to only one side - either the applique or the background. What is the name of your product? I think you used interfacing and it now stuck to your applique fabric. Warm it gently and try to pull it off. If it doesn't come off and you have no more fabric to start over with Steam a Seam or Wonder Under, then use one of those along with the interfacing. The applique will have more body, but in most cases that won't matter too much.