Vauxhall Signum engine turns over but wont start, was fine yesterday, no fault codes, no warning lights showing

I drove the car home yesterday, no issues other than as a auto it did not seem to change gear 2nd to 3rd, but seemed to sort itself, and drove fine. would not start this morning, engine turns , checked for fault codes , none found, no warning lights have come on, but I keep being told my right rear brake light is not working, but it is, any ideas , thanks