Is there a way to restore or at the very least input new data into a cell/day that was accidentally erased (this is speaking of the 'planned' training not 'actual')?
There isn't a way to restore data that's been deleted, but you can enter it manually by going to your Planned Blog and click the "Edit Today's Planned Training" link for that day. More detail can be found in the Planned Training Log section of the "Logging Your Workouts" help article.