This is often due to there simply being no connection between engine and propeller for one of a variety of reasons:
1. The propeller has fallen off.
2. The drive key in the flexible coupling has fallen out or has sheared, or the drive key in the propeller has been displaced/sheared. The first can be checked visually, and replaced.
The second at best will require a diver, at worst a haul-out.
3. The gearbox has seized. Little can be done without professional help.
4. The forward/reverse cable/link has become disconnected from the gearbox. Reconnect or operate the selection lever directly at the gearbox. This can cause manoeuvring problems in marinas and close to other boats!
