Car boot closed but not closed

[Debbie2014]

I have a qashqai 67 plate. I locked the car last night and it was absolutely fine. Went to the car this morning and on the screen on the dashboard, it was showing that the boot was open. Checked the boot and it’s shut properly, but the car still thinks it’s open. Tried opening and closing it numerous times, car still thinks it’s open. Theres nothing blocking it, the car just thinks the boot is open when it is definitely shut. Major problem is, as the car thinks the boot is open, the doors won’t lock with the fob. I’ve managed to lock the car doors with the actual key, but that doesn’t lock the boot. And the lovely Glyn Hopkin want to charge £250 just to tell me what’s wrong…
Does anyone know what it could be or where the sensor is??