ChildID | ParentID | Data2 | InputDate
 ------------------------------------
  1      |  345     |   100 | 3-5-2016
  2      |  345     |   0   | 3-12-2016
  3      |  345     |  150  | 3-19-2016
  4      |  345     |   0   | 4-20-2016
 ... more children with different parent IDs

Option Compare Database

Sub SoldOut()
On Error GoTo ProcError
Dim rs As DAO.Recordset
Set rs = CurrentDb.OpenRecordset("MainDataTable") 'This is the flat file version of the data above

'Check to see if the recordset actually contains rows
If Not (rs.EOF And rs.BOF) Then
    rs.MoveLast
    rs.MoveFirst 'Unnecessary in this case, but still a good habit
    Do Until rs.EOF = True
    GetLastWeek = 0
        If Not rs("Data") > 0 Then
            rs.Edit
            rs("Data") = GetLastWeek(rs('ChildID'), rs('ParentID'), rs('Data'), rs('InputDate'))
            rs.Update
        End If

        'Move to the next record. Don't ever forget to do this.
        rs.MoveNext
    Loop
Else
    MsgBox "There are no records in the recordset."
End If

ProcExit:
On Error Resume Next
rs.Close 'Close the recordset
Set rs = Nothing 'Clean up
Exit Sub

ProcError:
    MsgBox Err.Description
    Resume ProcExit

End Sub

Private Function GetLastWeek(ChildID, ParentID As Long, InputDate As Date) As Integer
    'given a record it looks up the weeks before and returns it if it exists
    Dim rst As DAO.Recordset, strSQL As String, rc As Integer ' SQL Code for seeing if last week's data exists.

    strSQL = "SELECT * " & _
    "FROM MainDataTable " & _
    "WHERE MainDataTable.[ParentId] = " & ParentID & "AND MainDataTable.[InputDate] <# " & InputDate & "AND NOT MainDataTable.[Data] = 0
    ORDER BY MainDataTable.[InputDate] DESC;"

    Set rst = CurrentDb.OpenRecordset(strSQL): rst.MoveLast: rst.MoveFirst
    rc = rst.RecordCount
    If rc = 0 Then GoTo Cleanup 'if no record, then we are out of luck
    If rc > 0 Then 'If there's some Record
        Do Until rs.EOF = True Or GetLastWeek > 0
            Dim price As Integer: price = rst("Data")
            If price > 0 Then: GetLastWeek = price
            rs.MoveNext
        Loop
    End If

Cleanup:
    rst.Close
    Set rst = Nothing
    If GetLastWeek = 0 Then GetLastWeek = 1 '1 means no data was found
    'Set so the output if nothing is found to 1 so that the code doesn't have to run on the same rows every single week
End Function