For future reference please use code tags around code that you submit.

Private Sub Workbook_BeforeClose(Cancel As Boolean)
Dim ws As Worksheet:    Set ws = Sheets("CBI Datasonde Cal")

If ws.Range("D5").Value = "" Then
    Cancel = True
    MsgBox "Please fill out Datasonde Make", vbCritical
ElseIf ws.Range("G5").Value = "" Then
    Cancel = True
    MsgBox "Please fill out Datasonde Model", vbCritical
ElseIf ws.Range("J5").Value = "" Then
    Cancel = True
    MsgBox "Please fill out Serial #", vbCritical
ElseIf ws.Range("M5").Value = "" Then
    Cancel = True
    MsgBox "Please fill out Station", vbCritical
ElseIf Application.WorksheetFunction.CountBlank(ws.Range("D9:D12")) > 0 Then
    Cancel = True
    MsgBox "Please fill out all blank cells under PRE-DEPLOYMENT CALIBRATION", vbCritical
ElseIf Application.WorksheetFunction.CountBlank(ws.Range("E41:E44")) > 0 Then
    Cancel = True
    MsgBox "Please fill out all blank cells under PREDEPLOYMENT MAINTENANCE/SETUP", vbCritical
ElseIf ws.Range("H41").Value = "" Then
    Cancel = True
    MsgBox "Please fill out all blank cells under PREDEPLOYMENT MAINTENANCE/SETUP", vbCritical
End If

End Sub