this happened to my SC-540. network board got fried after lightning hit a nearby tree.
the ground or some residual shock found its way through my router [probably through my dsl modem], and silently fried the network cards of all equipment connected to said router. pc network board/printer board/modem/router all had to be replaced. my SC-540 network board set me back about $1k.
i don't have an sp300, but i bet the pc/printer connection board is the culprit here.