Hello Firstcall,
Thanks for your post and welcome to the QuickBooks Online Community!
To identify the missing period of transactions, reconcile the account using the reconcile tool (Cog wheel > Reconcile). Reconcile each month separately, one bank statement at a time until you get to the date when the transactions stop recording.
Once you have this date, close the reconcile and go to Accounting > Chart of Accounts > drop-down beside the bank account and select the filter icon on the left hand-side. Set the filter to search for transactions from this date to the current date and apply. Transactions will show in date order so you can view here the date the transactions then began recording again.
You can then import the missing transactions following our guide here.
Thanks