Is it ok to go to a show when your horse lost a shoe the day before?

No, have the shoe put back on
In Portugal, if it's an oficial competition you can't show with one shoe off.. in Show Jumping you have to have all four shoes or no shoes at all. In Dressage you can have shoes only in the front hoofs, on the four or no shoes!
Personally I wouldn't show or I would ask the ferrier in the show to put a shoe! There's a lot of things that can go wrong riding without a shoe...
Hope it Helps
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