Okay, see file attached.

This checks 3 cells to ensure they're not empty.
If any are, it gives you an error message and you can't save the sheet.
Similar events can happen before printing etc.

You could also easilyget the code to check for the type of input.
CellContents = Sheets(1).Range("B3").Value
If CellContents < 100 then
.....
.....
Cheers,

Paul