So I lost that little card with the barcode they give you to bring the AP test and my test is tomorrow. What should I do?
Nothing?
Your school should hand you your AP booklet (as you need it to place a barcode on your new test), and you can just copy your AP number from the booklet onto another sheet of paper (after the test, of course) which you can then use to retrieve your scores.
Generally, I don’t believe that AP coordinators give the booklets to students before the test, but most generally allow them to keep the booklets after their last AP test. Thus, you could just ask to take the booklet home, and you have another 10 barcodes to use as insurance in case you keep losing cards (and barcodes).
Also, there’s nothing that states that you can only keep your AP number on that card… You could just create a notepad document with the number, as well.
However, if you were given the booklet and then lost it, that’s an entirely different matter.