We had some really hot days... 95+ degrees and unfortunately, my car had to sit in this, windows up, for several days. I dont know if it was from the heat or not, but now my headliner is loose, and almost hits my head on the driver side, and I'm only 5'8" tall.
I think with a MY2000 and 80,000 miles I'm beyond all warranty claims, unless there was a recall for this issue.
Assuming there is no recall, what is the best DIY fix?
Have you had your windshield replaced? I have been a windshield repair/replacement technician and have learned that it is very easy to cut the headliner loose near the windshield when cutting it out of the car. If you did have the windshield replaced, they probably cut it. If it's important to you to fix, have them replace your headliner for free since they were the ones who damaged it.