I need help with question mark on my iMac

To follow up on Limnos's reply, if the computer will not start up after using the trouble shooting steps in the link he provided, please make an appointment at your local Apple Authorized Service Provider to have the computer professionally diagnosed. It may be failed hardware. To locate and Apple Authorized Service Provider in your area please click Apple Authorized Service Provider Locator (AASP) & Genius Appointment

Read what to do "If your Mac starts up to a question mark", click here --> https://support.apple.com/HT204323