Why on earth didn't you deleted when you refound it on your computer?

Instead you chose to keep it, and you didn't tell her about what you found. That might be the most upsetting part - that while it might be an OLD video... you found it, and decided to keep it. And that you then didn't tell her what you found. So even though it's an old video, that's irrelevant now really as you chose to keep the video in the present.

I'm confused with how you say you haven't had any contact with her... but responded to a message in Snapchat? That part really confuses me, because that means that you have contacted her recently. Recent contact, plus a rediscovery of a sex video? Yeah... that does seem suspicious.

That said, give her time to cool off. If she is relatively calm, try to retell her your version of what went on. She is definitely flipping out a lot, to the point that she might not be hearing you at all.