Likely, you had an error and did not catch the error in the code and re-enable events again. Rather than closing and re-opening Excel, you can easily fix that by pasting this into your VBE's Immediate window and pressing Enter key.
Application.EnableEvents = True
Here is how I would do your Change event.
Private Sub Worksheet_Change(ByVal Target As Range) Dim r As Range
Set r = Intersect(Target, Range("A1:Z53"))
If r Is Nothing Then Exit Sub
Application.EnableEvents = False
MsgBox "A Cell changed in: " & r.Address
Application.EnableEvents = True
End Sub