Validate Cell as non-zero before running macro

Kevin, I think you mean

If Cells(1,1) = "" or Cells(1,1) = "0" Then
   Cells(1,1).Select
   MsgBox "Please fill in a non-zero value for A1"
   Exit Sub
End If

Fantastic responses guys.  Thanks!  Kevin, your solution inspired this solution.  The "exit sub" command was the missing piece for me.  Here's my finished solution with a twist...  I'm open to suggestions if you see a better way to do this...

If ActiveSheet.Range("T7").Value = "Job Pull" Then
    'this is only tested if value of T7 is "job pull". Other values skip this whole routine.
    If Worksheets("Master Inventory").Range("C1").Value = 0 Then
       'tests the "job number" cell which is on a different sheet called "master inventory".
         Worksheets("Master Inventory").Range("C1") = InputBox(Prompt:="Enter a Job Number",_
              Title:="Missing Job Number")
                  'If job number cell test is zero, an input box opens that will take their input job number
                  'and fill in the job number for the user in C1.  
         If Worksheets("Master Inventory").Range("C1").Value = "" Then
            Exit Sub
              'This if statement tests for the user hitting cancel which returns a null...if null, then exits macro.
        End If
    End If
End If

If ActiveSheet.Range("O7").Value = "Job Pull" Then
     If Worksheets("Master Inventory").Range("C1").Value = 0 Then
     'double-checks to make sure user didn't put in zero as a job number and hit OK in the last input box.
      'I couldn't find a way to nest in with former if statements.  But it works fine this way...
           Exit Sub
             'exits macro if job number is still zero
    End If
End If
    'At this point there will be a non-zero job number if user is creating a "job pull", so the macro will
    'continue normally as though the job number had always been there.
'rest of macro code continues here...

(same code as above, without all my distracting comments):

Sub TallySubtraction()
If ActiveSheet.Range("O7").Value = "Job Pull" Then
    If Worksheets("Master Inventory").Range("C1").Value = 0 Then
         Worksheets("Master Inventory").Range("C1") = InputBox(Prompt:="Enter a Job Number",_
             Title:="Missing Job Number")
         If Worksheets("Master Inventory").Range("C1").Value = "" Then
            Exit Sub
         End If
    End If
End If

If ActiveSheet.Range("O7").Value = "Job Pull" Then
    If Worksheets("Master Inventory").Range("C1").Value = 0 Then
           Exit Sub
    End If
End If

oops, T7 became O7, 1st comment to 2nd comment...  but you get the idea...

That works. But why stop there? Does a job number have to be numeric and be a specific length? Assuming it has to be a number between 1000 and 9999:

    If Len(Worksheets("Master Inventory").Range("C1").Value) <> 4 Or Not IsNumeric(Worksheets("Master Inventory").Range("C1").Value) Then

Kevin

Cool.  I didn't know that you could test for input value length.  That part will work for me.  Our job numbers have an alpha prefix of a letter, so the Not IsNumeric idea won't apply.  Thanks for your help!  I'm about 3 weeks into vba, so it's much appreciated.

Assuming you want a length of five with the first letter an alpha and the rest numbers:

    If Worksheets("Master Inventory").Range("C1").Value Like "[A-Z]####" Then

Kevin