Now that she probably has some idea of what is going on, you should prepare to discuss this topic with her in a way that makes her understand. It is probably best to have this conversation with her sooner rather than later so that she can get the right information.
Approach her on the issue. because you dont want her to develop any ideas. Also, talk to her about the birds and the bees but do it so that she understands its very normal
Yes definately talk to her, don't leave it too late as it will become a bigger problem. She's going to be very embarrassed, confused and upset so it is up to both of you to explain to her about sex, love and how that was a private moment she's wasn't supposed to see. Ask her if she has any questions and how she feels about it and apologise to her for leaving it somewhere where she could find it.
That's a pretty explicit thing to see for any child but for a child to see their parents in a video like that is quite shocking. It's important to explain that sex between to adults who love each other is not a bad thing or wrong. She will be grossed out for some time I think and it might have frightened her, don't put too much pressure on her to talk but be ready for changes in her behaviour for a while.