Did you convert your file from QuickBooks Desktop to Online? Reconciled transactions from QuickBooks Desktop will appear reconciled after the conversion process.
It's also possible that you have another user that did the reconciliation. You can review the Audit Log to verify this activities:
If you want to undo the reconciliation, you might want to do it one transaction at a time. Before that, I'd recommend reaching out to your accountant to verify if you'll need to do this.
Let me know if you need anything else.