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Disconnecting or deleting your bank from QuickBooks Self-Employed will wipe out all the data associated. Hence, your transactions from the last year are no longer available.
I'd recommend reconnecting the bank account to get back your bank transactions. Another alternative is to use a CSV file from your bank's website. You can import this to QuickBooks Self-Employed to bring back your transactions. You can use this link for the details steps: Add older transactions to QuickBooks Self-Employed.
Once you've retrieved your bank transactions, you can already categorize them. You can also set rules to speed up the process if necessary.
Please let us know if you require additional help with this.