Questions about what happened to Giselle when she vanished

[Twintails]

I'm referring to the part where she and Michel were watching the spring snow when they both went back in time. Giselle had a pained look after that scene. A few events later, she suddenly disappeared and Michel couldn't talk to her anymore. Was there an explanation why she was suddenly gone?

Apologies if I missed it, but I've been searching the Internet and couldn't find a straight answer.

[Magatsuhi]

She had exhausted her capacity. To be accurate, her soul had already been worn‐out since when she rescued Michel escaping from the spell of Morgana.

[Twintails]

引用自 Magatsuhi：

She had exhausted her capacity. To be accurate, her soul had already been worn‐out since when she rescued Michel escaping from the spell of Morgana.

Thank you. It's been bothering me for a while. I regret passing on this several times but now it's one of my favorite stories.