Updating CPU i5-7500 (3.4 GHz) to i7-7700K (4.2 GHz)

Hi there,

I have tried upgrading my iMac 2017 with a new CPU today (i7-7700K). Upon finishing and booting up there seems to be a problem.

the screen and fan both seem to not power on, with only one green LED shown in the following attached picture:

Does anyone have any experience with this problem? Is it a logic board issue? PSU? I feel like it is a logic board problem but cannot be sure.

Any help will be much appreciated, thank you for your time.