All mods were made years ago and nothing to the handlebars. I should clarify now that with the key in the ACC position only the gauge lights up - no headlight or running lights. Also, I recall that this whole problem happened when I pushed the START button trying to start the engine. Everything went dark.

 

Sad to say your last words probably are the main clue. It would be so much easier if the main fuse had blown and nothing worked at all! However much hangs on the wiring to the starter, so it is worth tracing the wiring from your starter switch, to see if there is a wiring problem, such as a short circuit. A multimeter is the way ahead - or a friend who knows how to use one! I had a serious problem with a bike some years ago, when the engine would cut occasionally when using the right indicator lights. Took a telephone engineer pal of mine, with umpteen years of wiring experience, to chase down the cause. That's all I can offer, peering across the Atlantic!