Unknown Bill to Customer

Hello,

During a training, a user has created a customer. The next day, we found out that the bill to customer of all the existing customers changed to the customer created. We really don't know what triggered the change. Even on the Sales and Receivables Setup, the field Parts inquiry default Cust No. got updated.

Thanks to advise, we've been trying to found out how the fields got updated since last week.

Regards
Bhavna