iMac won't start up after upgrade to new os

Please follow the steps in Startup Screens Troubleshooting Steps for that.

If you use a Mac that has a hard drive or a Fusion Drive and that drive is getting old then the stress of updating on rare occasions can cause that drive to fail. You can try restarting in the Recovery Partition (hold down Command + R on startup) open Disk Utilities - First Aide and run First Aide 2-3 times. Then try restarting again.

Hopefully the steps will work, however it they do not please contact your local AASP to have the computer diagnosed.

Is there any sound, lights, or action at all?

Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold ...

Command (⌘)-R

Installs the latest macOS that was installed on your Mac.

Can you please explain in some detail what is and is not happening? Only then can someone offer specific advice as to what you should do. Just guessing, but it may be that your OS upgrade has gotten corrupted during its download. This often happens if Wi-Fi is used. did you, or can you, use a direct connection via Ethernet? I would put the prior install app into the trash, do another clean download and see how that goes.