QB Issue Resolution:

If you did a backup right before you hit the undo button, you can restore the backup. Otherwise, you will have to redo the bank rec over again.

Resolution for Issue 'I accidentally clicked undo is there a way to go back so that I do not have to reconcile 151 items again' available: Yes (Solved).
Source: Intuit Community forum.

I accidentally clicked undo is there a way to go back so that I do not have to reconcile 151 items again: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for 151 items again
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Error Recovery for 151 items again [Canada]
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Additional Support Options: