Ran out of petrol put fresh petrol in and now it wont start

I think that your fresh fuel was full of water. I have heard that gas is usually good for about 30 days. The biggest problem is not the gas but the condensation in a gas can that is slowly getting emptied. This condensation builds up on the side of the gas can and drips down and into the gas. Pyshics makes water heavier than gas so the water will always be at the bottom, same in your mower tank, same in your mower fuel bowl.
Get a clear container and remove fuel line from the carburator, empty the fuel tank completely. Remove the bowl off the bottom of your carburator CAREFULLY so as not to damage the round gasket. Empty and wipe the bowl. Replace the clean bowl, fill with fresh gas and you will be able to proceed again. Before I wish you luck, look at the fuel you emptied into the clear container. If you see something at the bottom of all that gas, that would be your water problem. I hope this will help, GOOD LUCK.