Black screen imac 2017 21.5 inch after ram upgrade

I recently upgraded my imac from 8gb to 32 gb and the screen is black without a login box, blinking cursor, mouse arrow nor apple loading logo on switching it on. I put back in the original ram and it is showing the same black screen. I used a usb c hdmi adaptor to link the mac to an external monitor. it is showing a Mojave desksaver graphic, without the login dialogue box. the mouse is moving around the screen. There is nothing populating the screensaver other than the mouse arrow. When I press keys on the keyboard the mac makes the wrong password type of noise. Can anyone please advise.


Warranty is not an option. thank you.

Mac is 2017 new version. 21.5 inch. Mojave os.