Program not responding -excel vba for loop

There are a number of reasons why this code could have problems:

1. This line could fail if it's an old or compatibility mode version of Excel : wk.Range("N900000").End(xlUp).Row.
2. You are writing every cell individually which is very time-consuming. If Sheet3 has a lot of rows in it then your code could appear locked because it's taking so long to write
3. Your declarations have ceded control of types because all the 'untyped' declarations are Variants. This makes debugging very difficult. In your comment you ask "is it necessary?". Answer: not critical, but it will increase your debugging task by an order of magnitude and the code might work in ways you don't expect. In truth, a practical answer is "yes, it's very necessary".
4. There are no checks of the cell values and types. If cells are empty or not dates, your code will still run, And if all your variables are Variants, your code will aggregate incorrectly when you run Month(dt).
5. Using the .Text property can cause problems. If for example the date column is too narrow and you have #### in the cell, then that will be the .Text value (again, out of your control if your variable is an 'undeclared' Variant. Better would be Cstr(cell.Value) or Cstr(cell.Value2).
6. Your code is very inefficient because it loops through the same data in Sheet1 over and over again. Far better would be to load that just once into a collection whose key is the string value that you are testing. I haven't done that in the sample below as I'm a bit short of time but you should look into doing it. If Sheet1 has a lot of rows then your code really will be slow.

The other point is that it's far quicker to write an array to the Worksheet rather than one cell at a time. In your case, the month aggregations are ideally suited to an array. So you could optimise and shorten your code by using one. The code below deals with the points above and uses an array as an example for you.

You also seem a little unclear about the Debug.Print suggestion made by Noam Hacker. It's a good suggestion so I've given you a couple of examples of it in this code:

Public Sub SumByMonWithArray()
    Dim startRowA As Long, startRowB As Long
    Dim finalRowA As Long, finalRowB As Long
    Dim strA As String, strB As String
    Dim m() As Variant
    Dim dt As Variant
    Dim r As Long, c As Long
    Dim i As Long, j As Long

    'Define the start and end rows of each sheet
    startRowA = 2
    startRowB = 2
    finalRowA = Sheet3.Cells(Sheet3.Rows.Count, "A").End(xlUp).Row
    finalRowB = Sheet1.Cells(Sheet1.Rows.Count, "N").End(xlUp).Row

    'Dimension your array
    r = finalRowA - startRowA + 1
    If r < 1 Then Exit Sub 'exit if there's no data
    ReDim m(1 To r, 1 To 12)

    For i = startRowA To finalRowA

        Debug.Print "In loop i=" & CStr(i) 'shows progress (delete after testing)
        strA = Trim(CStr(Sheet3.Cells(i, "A").Value2))

        'If test value isn't blank run the comparison
        If strA <> "" Then

            r = i - startRowA + 1

            For j = startRowB To finalRowB

                Debug.Print "In subloop i=" & CStr(i) & ", j=" & CStr(j) 'shows progress (delete after testing)

                strB = Trim(CStr(Sheet1.Cells(j, "N").Value2))

                'If there's a match aggregate the month array
                If strB <> "" And strA = strB Then
                    'Populate a Variant with cell value and check it's a date
                    dt = Sheet1.Cells(j, "T").Value
                    If IsDate(dt) Then
                        c = Month(dt) 'Gets the column index of the array
                        m(r, c) = m(r, c) + CDbl(Sheet1.Cells(j, "S").Value2)
                    End If
                End If

            Next

        End If

    Next

    'Write the aggregate array to Sheet 3
    With Sheet3
        .Cells(startRowA, "B").Resize(UBound(m, 1), UBound(m, 2)).Value = m
        .Activate
        .Range("A1").Select
    End With
    Application.ScreenUpdating = True

End Sub

Consider this mock-up data in Sheet1:

First add a column to the right of column T (Date of Sales?) with formula =MONTH(T2) for cell U2.

Add/Change the Monthly label to Integer (B1:M1 in sample).

Then create dynamic named ranges:

• SalesItemCol with formula =OFFSET(Sheet1!$N$1,1,0,COUNTA(Sheet1!$N:$N)-1,1)
• SalesQtyCol with formula =OFFSET(Sheet1!$N$1,1,5,COUNTA(Sheet1!$N:$N)-1,1)
• SalesMonthCol with formula =OFFSET(Sheet1!$N$1,1,7,COUNTA(Sheet1!$N:$N)-1,1)

Finally on B2, use formula =SUMIFS(SalesQtyCol,SalesItemCol,$A2,SalesMonthCol,B$1) then auto fill the rest.

Alternatively you can create macro to do the above...