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There may be dirt, or even rubbed-off ink from the printer film, on an optical sensor that relates to the printer film and is located in its area inside the printer. So... if it doesn't waste the film... remove the film carefully so that it might be re-used, set it aside, and clean (with the power Off, and unplugged, and using a clean, dry, lint-free rag cloth) inside the printer where the film is located. Then replace the film carefully, close up, see if it now works when plugged in and turned On.