Stay calm. There are two ways to find your PowerUp Rewards account number:
First, you can find your number printed right on the back of your handy little black-and-red (or silver-and-red, if you’re a fancy Pro member) card. You’ll find two numbers printed back there, so be sure to enter the number just under the barcode, like so.
If you have activated your PowerUp Rewards account but can’t find your card, all is not lost! Go to your PowerUp Rewards Account page and login with the e-mail address and password you used when you activated your account. Once you're logged in, under the "Rewards Account" tab, next to "PowerUp Rewards ID" you’ll see your account number clearly displayed, as shown in this Rewards Account page screenshot.
Alternatively, if you’re logging in from the front page of the GameStop PowerUp Rewards site, you’ll arrive on the Dashboard. Mouseover the "My Account" menu item, and click on "Rewards Account" from the dropdown menu, as shown in this Dashboard screenshot. Now you'll be on your Rewards Account page, and next to "PowerUp Rewards ID", you’ll see your account number clearly displayed, as shown in this Rewards Account page screenshot.