Hi,
I've never had a full lung collapse before aka pneumothorax, just a portion.
When that happened it was very painful, but I just assumed it was a shoulder injury because I'd tweaked my shoulder a few weeks earlier rock climbing.
As Linds said, it was very painful, it felt stiff, I was more short of breath than usual. It just didn't feel like air was going in there.
They did a bronch and tried to re-inflate it but it didn't work. Hope you don't have this happen, because I lost quite a bit of lung function that I'm still getting used to.
Take care,
Chris