Find the name of the manufacturer.

I'm almost positive it's Anderson. My father-in-law built the house,
and I'm pretty sure that's all he uses. But we don't speak any more,
so I'll have to check it out.

I actually called a "window expert" called Superior Exteriors. The
owner of the company came out to look at it, and said it's impossible
to repair or replace (it's custom, so it's stained wood to match all
the other trim). You believe that? It's impossible to fix a window.
So what? Demolish the house?

He said there are usually strips to pull off and access the glass.
Then he left. I looked again and the strips are there in plain sight,
on the outside of the window.

Window expert.