1. Is this a 12 volt battery? Measure voltage across battery terminals when it is clicking when trying to start. Post that.
2. If the battery is 12 volt, try a jump start from a vehicle battery or charge up the battery with an external charger. If it starts then, what is the voltage across battery terminals while engine is running.
My guess - because it's a new battery and started earlier so you could mow - is that it isn't charging and you lost battery charge while mowing. The responses to my question will help establish if that is the case.