What you have here is a classic example of an infinite recurring loop.
What is happening is that in your line:

Target.Value = Target.Value + Range("A1") + Range("B1")

you change the value of a cell. Even though this is done through VBA, this counts as a change to the worksheet. And since this module is within the Worksheet_Change module, it catches this change and starts this same worksheet change loop. This obviously will cause the change to happen again, and the loop to start again until it causes a stack overflow and crashes Excel.
Edit: After some testing I found a workaround. It's by no means ideal but you can use a global variable to flag whether this loop has already run once this iteration:

Dim run As Boolean

Private Sub Worksheet_Change(ByVal Target As Range)
Dim lRow As Long, i

If run = True Then
    run = False
    Exit Sub
End If

lRow = Cells(Rows.Count, 12).End(xlUp).Row

    If Target.Cells.CountLarge > 1 Then Exit Sub

    If Not Intersect(Target, Range("L20:L" & lRow)) Is Nothing Then
            If Not IsNumeric(Target.Value) Then Exit Sub
            run = True
            Target.Value = Target.Value + Range("A1") + Range("B1")
    End If

End Sub