Yes. Q-Park.fr customers can enter and leave the car park 24h/24 even if the car park access is closed for the public. Just press the call button at the car park access and communicate your reservation number. The ticket issued at the car park entrance will also give you access to the secure pedestrian access gate.
You will find all the information you need (procedure, reservation number, QR code, pedestrian access code, etc.) in the reservation confirmation email.