Hello Gar-Bob,
Thanks for reaching out here on the Apple Support Communities! I understand that after restarting your iMac all of the icons changed position in Lauchpad. If this is correct, let's begin with some basic troubleshooting, if you haven’t done so already, and then work our way toward more complex ones. This will help us to isolate the issue further. Be sure to check and see what happens with the arrangement of the apps after each step.
1 - Restart your iMac: Go to the Apple menu in the top left corner of your screen > Restart.
2 - If the issue persists, restart in safe mode and test again: Use safe mode to isolate issues with your Mac.
3 - If still occurring in safe mode you’ll need to create a new user and test it out again. This article will show you how: How to test an issue in another user account on your Mac.
If you still are experiencing the same issues with the app arrangement in Launchpad, or if you’re seeing any error messages, let us know.
Cheers!