OC has said that she died in a climbing accident, which makes this more of a PSA than anything else, but sudden happiness (even/especially if it seems geniune) after a long period of depression can be an indicator of suicidality. Generally it’s because they’ve decided to go through with it, so they feel at peace.
That said, being from an accident doesn’t make her death any less tragic. I hope her loved ones are making it through okay.