The only way to revert the schedule information to what it was prior to clicking the Generate master schedule button is to use the Snapshot functionality. If a "before" snapshot was created prior to generating the master schedule then you need only "Restore" that state:
  1. Navigate to onRecord > Procedures > Schedule Maker
  2. Select the appropriate year and school level from their corresponding filters
  3. Click the Scheduling Snapshots link in the floating navigation on the left side of the screen
  4. Click the Restore snapshot button
  5. Click the Confirm button to complete the restoration