2019 iMac chimes on startup, screen stays black, then fan starts

Hey, lstoddart, i presume you’ve tried this /done this already:

Reset NVRAM on your Mac - Apple Support

usually i do the Pram thing at least 3 times/3-4 chimes thrb let go of all keys and let the Mac start up/boot up as per usual…,if you tried/did that already, no luck/no change… then you could try “safe mode”… reboot, holding down “shift” key on your keyboard until you see “safe mode” on your monitor screen, then let go… still no change/no bueño… try diagnostix mode.. reboot, hold down “D” key on keyboard, be patient as it’s doing some background tests… keep holding D key… you should see some test results eg “pass/fail” if all passed OK then call Apple Support via their 1-800 number in your country… they also have a chat feature… be polite, be patient and be at your Mac when you call/contact them… you might have to take it in to an Apple Store with Genius Bar or AASP ( apple authorized service provider) for an evaluation. Call/contact either one , make appointment and take ‘er in and see what they have to say… at the Apple Store, the evaluation is always free…

Good Luck

John B

[Edited by Moderator]

Have a look at > If your Mac doesn't start up all the way - Apple Support

If nothing there helps, then you will need to schedule a service appointment at your local Apple Store or AASP.