By default, the add-in does not save simulation data after you close a spreadsheet. The values for analytics functions will remain in place if you re-open a spreadsheet, but they will become #N/A errors if you recalculate it.
In the Professional Edition, you can save simulation data with the spreadsheet. This can be useful if you have a particular data set you wish to preserve, or if you need to continue your analysis later.
To turn on this feature, click Advanced Properties on the Monte Carlo toolbar, and select the File tab.
Note that saving trial data for large models with many iterations can make the file much larger than it would otherwise be.