Dashboard lights as normal until you turn the key to start then everything electrical loses power. Total electrical failure. You have to disconnect the battery to get power back, but lose it again when you try to start.
Get your battery load tested .Some cars cut out non essentials if battery too low .Starting it could draw it down further until boosted again by alternator. Could try starting it with a jump start or run it for a while to see if it comes back on .
Thanks for the ideas, but maybe I should have given more information though. It first occurred while out and needed to be jump started by the recovery man (RAC) who measured the battery voltage at 5V. So got it back to mine and bought a new battery and it started first time, but second time a couple of hours later it happened again. Battery out, re-charge, and the same thing. Starts first time, but not after a couple of hours standing.