Heavy or wet grass may have caused some extra load on the battery and a wire inside the battery has heated up and perhaps melted its insulation. It is possible that this has occurred somewhere else in the mower but the battery is the most likely.
It is fairly easy to open the battery casing to check the wiring but it should only be done at your own risk. The wiring is simple but a short circuit between battery terminals is enough to melt the metal of a screwdriver, more than capable of giving a nasty burn or a shock.
To open the cover place the battery sticker side down on a bench and remove all the screws except for 2 which hold the external battery connection in place. Before lifting the shell off you should flip the whole battery over. The screws will fall out onto the bench but the internal parts will stay in place in the shell.
The link above shows the type of fuse inside the battery. Please take a look at this, these fuses sometimes break but leave a small bridging connection. Please also check that each of the connections to the top of the battery is tight and the wires are not damaged.
Please take photos of any damage and email them to me.
If you can test the voltage of the battery you should expect 24V DC from the pack as a whole or 12V from each of the 2 cells inside.
If you can identify a fault perhaps we can send you spare part