Originally Posted by KiwiJochen

You will have burnt a Mode1 CD. Trying to load OS from a Mode1 CD puts a Mk3 nav in an endless loop.

But you can recover from this.

You can recover by burning a Mode2/Form1 CD. Use ImgBurn to confirm the image type is Mode2/Form1 before you burn the image.
Verify the OS CD after the burn that it is error free, and that it is a Mode2/Form1 CD.

Then insert the CD into the nav and it will load the OS and come back to life.

This problem has happened to many people, they always managed to fix it with the Mode2 CD.