If your bank account is connected to QuickBooks Online, it's possible that the deposit is not added to the bank register. You can check the Banking page to find the transaction.
On the left pane, select Banking.
Select the For Review tab.
From the Funnel drop-down, enter the date.
If the transaction is not there or if you're not using bank feeds, it's possible the transaction is deleted. Let's check the audit log to see the changes made to your books.
Click the Gear icon.
Select Audit Log.
From the Filter drop-down, choose the appropriate Date and Events filter to narrow the results.
If you get the same result, it's possible the deposit was not recorded last year. You can enter that in QuickBooks Online (QBO) by using the Bank Deposit feature, but this will affect your books for 2018. I suggest consulting an accountant for more ways to record it.