That model or a similar appearing model has the battery cover/cowl snap/clip on, so maybe try wriggle it a bit, as any debris shouldn't impede removal too much: the grooves may be spring detents, and the holes could have screws down in them, or simply plastic clips. then to put it back on, clip it back into place & check that its held on ok.
Please be certain that its unplugged when working around it or on it, in addition to being switched off! Its very possible that over time vibration has caused a connector to come loose or break, or a wire has broken etc. Also, on cordless models, the batteries are held in with a tiewrap.., but the batteries are probably available, similiar or interchangeable with back up pwr supply types same as for PCs etc, generally four 12v units with connector clips onto the terminals. Please be careful working around it, until the batteries are disconnected, it could spin etc, same for when reconnecting.
If it stopped abruptly, could be a connector around the motor, or controls, or a broken wire, or possibly the motor itself. eg, over time, vibration could have loosened a connector, worn a wire through, or terminal failure, or connector on the motor, or corrosion.
Also, maybe double check that the pwr cord is connected ok and that pwr is reaching the outlet the pwr cords attached to, & is reaching the mower.