I've experienced just this problem (SXS500) over the last couple of days - turn key to first position, full display as usual, fuel pump starts up, check lights working, yes - on, then off - then attempt to start .... click from somewhere around the battery (?), screen goes completely blank, lights won't come on; everything dead. Disconnect battery (earth lead), wait ten minutes, reconnect, all looking good, screen working etc., but exactly the same result. Eventually followed the advice above and connected a backup battery to the existing battery and, voila, everything burst into life. So I popped into town and bought a brand new equivalent, optimistically screwed both terminals on tight, re-installed the cover, and away she went, and has been absolutely fine since ...... I've never seen a battery fail in this way (and my meter shows 12.65 volts even now). Battery was a Yasuda made in USA, but the original I think (2016), and clearly had developed some internal fault (?). So no problem with the machine itself, which, apart from a starter motor replacement (thanks to this forum for installation guidance), has been faultless. Bought secondhand at auction, sight unseen; foolish perhaps, but relatively inexpensive!