SelectionChange event not firing

Respectfully, there is no way the code you posted ever worked. Target(c).Value and PreviousValues(c) make no sense at all if c is a Range object, in addition to the errors I mentioned previously.

You could do something like this:

Private Sub Worksheet_Change(ByVal Target As Range)
    
    Dim LR     As Long
    Dim x As Long, y As Long
    Dim a As Range
    Dim sel As Range
    Dim PreviousValues As Variant
    
'    On Error Resume Next
    Application.EnableEvents = False
    Set sel = Selection
    Set a = Target
        PreviousValues = a.Value
        Application.Undo
        If a.Count > 1 Then
        For x = 1 To a.Rows.Count
            For y = 1 To a.Columns.Count
                If a.Cells(x, y) <> PreviousValues(x, y) Then
            With Sheets("AuditTrail")
                     LR = .Cells(Rows.Count, "A").End(xlUp).Row + 1
                    .Cells(LR, "A").Value = Now
                    .Cells(LR, "B").Value = Environ("UserName")
                    .Cells(LR, "C").Value = ActiveSheet.Name & "!" & a.Cells(x, y).Address
                    If PreviousValues(x, y) = "" Then
                        .Cells(LR, "D").Value = "BLANK!"
                    Else
                        .Cells(LR, "D").Value = PreviousValues(x, y)
                    End If
                    If a.Cells(x, y).Value = "" Then
                        .Cells(LR, "E").Value = "BLANK!"
                    Else
                        .Cells(LR, "E").Value = a.Cells(x, y).Value
                    End If
            End With
        End If
        Next y
    Next x
    Else
                 If a.Value <> PreviousValues Then
            With Sheets("AuditTrail")
                     LR = .Cells(Rows.Count, "A").End(xlUp).Row + 1
                    .Cells(LR, "A").Value = Now
                    .Cells(LR, "B").Value = Environ("UserName")
                    .Cells(LR, "C").Value = ActiveSheet.Name & "!" & a.Address
                    If PreviousValues = "" Then
                        .Cells(LR, "D").Value = "BLANK!"
                    Else
                        .Cells(LR, "D").Value = PreviousValues
                    End If
                    If a.Value = "" Then
                        .Cells(LR, "E").Value = "BLANK!"
                    Else
                        .Cells(LR, "E").Value = a.Value
                    End If
            End With
        End If
       End If
    Target.Value = PreviousValues
    sel.Select
Ending:
    On Error GoTo 0
    Application.EnableEvents = True
    
End Sub