Well, let's simplify it so it becomes obvious.
In a new file, if we write 1,2,3 into the first row...
...and run this code...
Sub Test()
Range("A1:C1") = ""
End Sub
...the values are gone, because your code overwrote them.
If you don't save the values anywhere, you won't be able to restore them, right?
BTW, Range("A1:C1").ClearContents is the correct way to clear the content of cells.
So, for your layout/idea, you need 12 more sheets in which you have to save the data for each month, then you can also restore them depending on which month you select in the drop-down.
This is of course time-consuming, so you can also make one sheet per month and leave the data there. No drop-down, no VBA, just a few formulas, done.
Andreas.