Imac 27 stuck on recovery 24:00

If you can't start up from macOS Recovery, try Option-Command-R

as per > About macOS Recovery on Intel-based Mac computers - Apple Support

If that doesn't work, then you may need to use the recovery partition that is on the old HDD.

1. Put the old HDD into an external USB enclosure and startup in macOS Recovery.
2. Put the old HDD back into the iMac and then startup in macOS Recovery.

What year model iMac?

What SSD did you use?

Hold the power button for 10 seconds to shut down the iMac.

Plug the iMac into your Router using an Ethernet cable.

Then startup the iMac in macOS Recovery.

Good work, glad to hear the problem is solved.