The computer may be asleep. Press one of the buttons on the computer to wake up the computer.
Batteries need replacement or charging
Is the computer too cold? Optimum operating temperature is 30-100 degrees Fahrenheit.
Battery contact inside the Joule may be depressed. Remove the battery cover and battery to expose the silver battery contact. There should be sufficient spring in this tab. If there is not, GENTLY pull up on the contact to reform it.