What could happen if an employee accidentally stole $5 gift card and used it?

I'm under 18. I didn't even know I did it but I accidentally took a $5 gift card from a customer who left it at the desk. I used it the same day which looks like I intentionally took it. I got talked to and told them I didn't remember doing so. It was as accident. I feel bad and I want to reimburse the guest personally. They told me they will get back to me but corrective action will be taken (even up to termination) as this is considered theft.