I would grab her hands while getting on my knees, reach for the engagement ring I always hide in my pocket, in case something like this happens, and propose to her.
If she said yes, I would immediately say "JKLOLZ"
If she said no, I would still say "JKLOLZ"
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What, you wanted a serious answer? Geez, FINE!
I would look at her with a puzzled face, ask her when did she get back from *censored city* and where did that come from.
Assuming she didn't suddenly disappear, I would probably spend the rest of the day with her.
If she did disappear, I would sit on my couch for the next 6 hours wondering what the hell just happened.