I had a similar problem with my Fiat Stilo earlier in the year. One day the car would start and next morning it wouldn't. Similarly with battery which was almost new. Took it back to dealer who tested it and said it was OK. Never did find out the real cause. My nephew who is a motor mechanic had a good look at it and the only thing he could find was corroded terminals on the starter motor. He cleaned up one of them but couldn't undo the nut on the other one, so it never got done. Bad positioning on the starter motor just under the front of the engine with no covering and gets all the muck thrown up on it. Bought one of those emergency engine starter battery packs. Don't have the car now.