I have created a user form that has a text box asking the user to enter an
employee ID number. Upon entry, the number is placed in cell C3 on the
spreadsheet. There is a VLOOKUP formula in cell C2 that goes to another
sheet (in the same workbook) and searches for the ID number and brings back
to cell C2 the cooresponding employee name.

If the number entered in the text box is not found then #N/A results in cell
C3.
I want to create a msgBox that would pop up as soon as an invalid number is
entered in the text box saying "Invalid Employee ID Number".

How do I get there?

My TextBox code is as follows:

Private Sub TxtEmployeeNumber_Change()
Range ("C3")=TxtEmployeeNumber.Text
End Sub