The disappearance of class tracking could result from software updates, integration issues, or changes in user permissions, Dawn.
To fix this, contact bill.com customer support for assistance.
You can pull up Sales by Class Details or Purchases by Class Detail reports to review transactions per class: Run reports by class in QuickBooks Online.
If you have any other questions or concerns regarding QuickBooks Online, feel free to leave a comment. We're always here to assist.