Sudden loss of power

[SallyID]

Hello, i have a 2018 diesel Micra with start stop. After a couple of 10 mile drives today i parked up at the supermarket and when getting back in the car there was no power at all...i had to use the key to get in. The key is still showing a red light when pressed. My husband jump started me and i got home but once the engine was switched off i couldn't re start and again no power at all. Battery, alternator or other problem? Thoughts please