Usually they'll have ypur name on a confirmation list. What you can do is check your SPAM and Trash folder for the confirmation email. Otherwise, the previous advice are great options as well.
Call the studio and see if your name is on the list for that day if you can't find your voucher. Sometimes they will send a reminder email.
I know i can't find mine. Of course my luck
When I was turned away they gave me a voucher for guaranteed admittence for another show.
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