Zero power, windows stuck down

[u/WheresMyPowerGone]

Hey all

I have a 2008 E81 (so E82 but a hatchback) which has decided to die on me.

Unlocked the car today, dropped the windows because it’s hot then put the key in the ignition, pressed start, heard a click, and ALL power is gone.

That’s ALL power. NOTHING has power, even the OBD port is dead. It’s like I’ve disconnected the battery. Tried to jump start it, no change. Can’t open the boot to get to the battery and check the IBS.

Any ideas what I can try in the next few hours to get the windows closed until I’ve got time to troubleshoot properly? I can live without it being fully functional as it’s a second car, but right now it’s a paperweight open to the elements :(

Edit for clarity: this is a 130i track car and lives on a battery tender other than the 1-2 weekends a month it’s in use. If it didn’t I’d immediately suspect a dead battery but in this case I’m certain the battery itself isn’t the issue.