It is likely that your blade boss has sheared off. The blade boss sits behind the blade and holds the blade on with two metal pins – one either side of the centre of the blade. These pins are designed to shear off and stop the blade from turning if the blade hits something hard like a stone. They shear off to stop the blade from turning and doing more serious damage to the engine or underside of the mower. If these have sheared off, you will need to replace the blade boss.
Blade bosses are inexpensive and can be purchased from most dealers online. They are easy to fit, and your dealer or manufacturer should be able to advise you how to do this. It is important to make sure however, that the blade bolt is tightened to the correct torque setting otherwise the new blade boss may shear when you restart your mower. Your manufacture can advise you of the correct setting.
If you are not confident to do this yourself, your local dealer should be able to help you with this.