Jeanke

I am assuming that your routine calls firstly the Text0_BeforeUpdate and
then Text0_AfterUpdate?

If so, you will get the problem you are describing as you are not telling
your calling routine that there is a problem and not to open the next form.
Setting the Cancel = True in the second routine will not achieve anything as
the AfterUpdate event does not support it.

If you want to do it this way, then I would suggest the following:

1. Remove the code you have in the AfterUpdate Event and place it in a
function that returns a boolean value.

In the code you have behind the button, remove the first check (as this will
be invoked automatically when you try to leave the field and setting CANCEL =
TRUE will stop the user leaving the field until they have entered 10 digits
(be aware that as you have it coded, once they enter something in the field
they will not be able to exit until they have entered 10 digits)

Then call your new function and decide whether to open the form based on the
response. Eg.

If CheckOrderExists then
  code to open order form
endif

Function CheckOrderExists as Boolean
 
  If IsNull(DLookup("[SAPnr]", "Order data", "[SAPnr] = '" & _
       Me.Text0 & "'")) Then
   MsgBox "Did Not Find PO  " & Me.Text0, vbExclamation, "PO number not
found"
   CheckOrderExists = False
   me.text0.setcursor ' set cursor back to this field
 Else
   CheckOrderExists = True
 End If

End Function

okay. you say that you enter an order number in a textbox, and then "have to
press a button to check two things". that doesn't make sense because the
first thing you listed is validation code that *automatically* runs on the
textbox's BeforeUpdate event. don't get me wrong, the code itself, and its'
placement, is fine - but it has nothing to do with any code that may be
running from a command button on your form.

same issue with the second thing you listed:  the code runs on the textbox's
AfterUpdate event (where it does no good, as Chris and i have already
noted). it has nothing to do with any code that may be running from a
command button on your form. the other problem with the code you posted for
#2 is that the "open form" code you posted is *outside of the sub
procedure*. (it comes *after* the End Sub line.) this doesn't make sense to
me, because code that's outside a procedure will not run (not to mention
causing an error during compile).

suggest you change the code in the textbox's BeforeUpdate event to:

Private Sub Text0_BeforeUpdate(Cancel As Integer)

   If Len(Me.Text0) <> 10 Or Not IsNumeric(Me.Text0) Then
       MsgBox "Must be 10 digits", vbExclamation
       Cancel = True
   ElseIf IsNull(DLookup("[SAPnr]", "Order data", "[SAPnr] = '" & _
       Me.Text0 & "'")) Then
       MsgBox "Did Not Find PO  " & Me.Text0, vbExclamation, _
           "PO number not found"
       Cancel = True
   Else
       Dim stdocname As String
       Dim stlinkcriteria As String

       stdocname = "Order entry form"

       stlinkcriteria = "[SAPnr]=" & "'" & Me![Text0] & "'"
       DoCmd.OpenForm stdocname, , , stlinkcriteria
       DoCmd.Close acForm, "change order prompt", acSaveNo
   End If

End Sub

if you *are* clicking a command button on your form, that you expect to do
something related to checking the PO number and opening the form, suggest
you post it here, so we can figure out how it's involved in all this.

btw, if the SAPnr field, in the Order data table, is a Number data type,
then you need to change both the DLookup() criteria and the stlinkcriteria,
as

DLookup("[SAPnr]", "Order data", "[SAPnr] = " & Me.Text0)
stlinkcriteria = "[SAPnr]=" & Me![Text0]

hth