Can you open the windows at all?
Before Central Air and A/C we used to have black out curtains on all the windows. We'd open up all the windows at night time when its cooler and close them before the sun comes up and close the curtains keeping the cooler air inside for the day, hope this helps.

No, the situation would be in the case of a total bug in & lock down where opening windows and doors is not an option - ie volcanic ash, security etc.

I agree with plainolme on the blackout curtains. I purchased these for my home this year, and it made a 3-4 degree Celsius difference during the afternoon.
A suggestion for providing airflow could be to rig a fan up to a bicycle sprocket. If you have a way to store blocks of ice, you can run the fan over them and it will get quite cold, or you can mist water to enhance the cooling effect.

I think I would see about making a vent hole (s) on the lowest part of the house. Preferable right behind some shrubs or bushes (preferably prickly/spiny ones) so it doesn't stand out.
Just a small one(s), so no one can crawl thru.
Put some sort of filter over it, so that it would keep out ash.
Then another vent up top, creating a chimney effect.
Cool air drawn in, will help push the heat upward to the top.

We used to do that at home.
Open windows in the basement and it would help to cool by pushing the heat out the upstairs windows and vents up there.
No such thing as AC way back then. 😉

Sorry Grace, thats all I can think of right now.

Cool water on a small towel around your neck can definately keep you cooler if your wondows are all shut indefinately where is your air going to come from?

I like the idea of vent holes with mesh down low and up top OTG, that would definitely help. We had similar in an old caravan we had and they worked quite well.

Plainolme, yes thank you, I thought about the wet towel, and OTG's idea would assist with fresh air into the house and expel warm air.

I would also have to think about my only source of cooking or heating food inside which is the woodburner and would make matters even worse....cold foods and drinks only for the duration I suppose. But I'd have to light the woodburner to heat water if it was absolutely necessary...don't like that idea much..phew!!!

Open up your attic entrance and let the hot air rise and escape through the roof vents. Hot air rises and will not fall into home. Give it a shot sometime in the summer. Yeah I know the roof gets warm especially with darker shingles.

USB/Solar fans are available, rechargeable batteries in battery fans, spray bottles, if water is adequate a damp towel if hung in a breeze thru path will give some cooling.. similar principle to an evaporative cooler, old school folding paper or plastic hand fans - cheap on ebay.. i bought 5 recently. we have had a huge heatwave here in Australia recently n i'm glad i'm ready if power goes.

maybe a rig that runs a fan manually - eg the bike sprocket AND charges usb items at same time.

Anywhere your blood is close to the surface of your skin it can be cooled. Wet wrists and neck or feet in water will do remarkable job of cooling you off.
In surgery the way they control your body temperature is by the temp of the air you breathe because there's so much blood exposed to air temperature in your lungs. If you can breathe cooler air it will cool your body quickly. Your A/C unit may not cool off your room but just inhaling the cool air will cool your blood and hence your body.

One thing that really worked for us this past summer was using emergency mylar bags over the windows. I just cut them open and taped them to the outside of the windows, primarily just the ones that the sun hits. They pretty much reflect 100% of all heat. And it blocks out most light too. We did this on our slider door, worked wonders. Just had to remember to wear shoes when your stepped onto the patio - almost burned your feet off! Another addition was to hose down the patio - helped the cool down when the sun went down.

The wet towel thing works for dry heat but not so well for high humidity areas. Think Brazil vs Arizona.

i built a solar heater on the outside wall that draws cool air off the floor and releases hot air back in the top.in the summer i shut the outlet and open the outside outlet this draws the cold air from the basement (on the otherside of the house) and puts it outside this gives quite a nice cooling air flow.the heater is empty brake clean cans painted black behind glass.cans open easy with canopener and tin snips just be careful of cans they are areasol but clean and they go in the trash anyway.

We have metal blinds on the outside of our windows that we let down from the inside...it really helps to keep it dark and cool

T he last thing on my mind in case of "shift" is being too hot.You must be joking.One time I Had to work in 45 C for for few wks and it was different but survivable because I had to.Are we really that spoiled? I rather worry about being very cold , being out of fire wood.
Henry