This could be a few things so we will need to go through a few different checks.

My first guess would be maybe your flywheel is loose.

Attached to the front of the engine is a large metal disc called the flywheel. This is held to the engine by a large nut that is under the clutch bell. If you turn the flywheel it should also turn the engine. If you can turn the flywheel with your fingers without it turning over the engine it means it has come loose and you need to tighten it.

You can see info on how to tighten it here - http://www.nitrotek.co.uk/help/changeclutch.html

Please carefully check this and let us know if the flywheel is not loose.