It may take two people....when you turn key, you should hear a click at the solenoid.
The click is the sound of the plunger inside being drawn down by the solenoid electromagnet, allowing gas to the fuel jet.
Possibly feel it with the click.
Put red lead of a multi-meter on the solenoid connection (other lead to ground) and see if it shows 12v when trying to start the mower
Check the connection of the wire to it....maybe it has been knocked off, or is cut.
You could hold a jumper wire from the red battery post to the solenoid and listen for the click of operation.
Or rig that up temporarily and try and start the mower (power wire from battery to solenoid)
I hope that all is clear.