Yes Peter is correct, mind you, in case this is relavent, pressing Ok, with no response will also return "". So to differentiate, more precisely...
If vbCancel = InputBox("Pick Number From ....",vbOkCancel) Then
....
or
Dim varResponse As Variant
varResponse = InputBox "Pick Number From ....",vbOkCancel
Select case varResponse
Case vbCancel
MgBox "You Must Reply by...
Case vbOk
Docmd.Open....
End SElect