from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(240, 50, 321, 121))
        font = QtGui.QFont()
        font.setPointSize(36)
        self.label.setFont(font)
        self.label.setObjectName("label")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 26))
        self.menubar.setObjectName("menubar")
        self.menuFile = QtWidgets.QMenu(self.menubar)
        self.menuFile.setObjectName("menuFile")
        self.menuEdit = QtWidgets.QMenu(self.menubar)
        self.menuEdit.setObjectName("menuEdit")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        self.actionCopy = QtWidgets.QAction(MainWindow)
        self.actionCopy.setObjectName("actionCopy")
        self.actionPaste = QtWidgets.QAction(MainWindow)
        self.actionPaste.setObjectName("actionPaste")
        self.actionSave = QtWidgets.QAction(MainWindow)
        self.actionSave.setObjectName("actionSave")
        self.actionNew = QtWidgets.QAction(MainWindow)
        self.actionNew.setObjectName("actionNew")
        self.menuFile.addAction(self.actionNew)
        self.menuFile.addAction(self.actionSave)
        self.menuEdit.addAction(self.actionCopy)
        self.menuEdit.addAction(self.actionPaste)
        self.menubar.addAction(self.menuFile.menuAction())
        self.menubar.addAction(self.menuEdit.menuAction())

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

        self.actionNew.triggered.connect(lambda: self.clicked("New was clicked"))
        self.actionSave.triggered.connect(lambda: self.clicked("Save was clicked"))
        self.actionCopy.triggered.connect(lambda: self.clicked("Copy was clicked"))
        self.actionPaste.triggered.connect(lambda: self.clicked("Paste was clicked"))
        
    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.label.setText(_translate("MainWindow", "TextLabel"))
        self.menuFile.setTitle(_translate("MainWindow", "File"))
        self.menuEdit.setTitle(_translate("MainWindow", "Edit"))
        self.actionCopy.setText(_translate("MainWindow", "Copy"))
        self.actionCopy.setShortcut(_translate("MainWindow", "Ctrl+C"))
        self.actionPaste.setText(_translate("MainWindow", "Paste"))
        self.actionPaste.setShortcut(_translate("MainWindow", "Ctrl+V"))
        self.actionSave.setText(_translate("MainWindow", "Save"))
        self.actionSave.setShortcut(_translate("MainWindow", "Ctrl+S"))
        self.actionNew.setText(_translate("MainWindow", "New"))
        self.actionNew.setShortcut(_translate("MainWindow", "Ctrl+N"))

    def clicked(self, text):
        self.label.setText(text)
        self.label.adjustSize()

if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())

Live example

class Window1(QMainWindow):
    def __init__(self, value, parent=None):
        super().__init__(parent)
        self.setWindowTitle('Message Window')
        self.setFixedWidth(500)
        self.setFixedHeight(600)
        self.setWindowIcon(self.style().standardIcon(QStyle.SP_FileDialogInfoView))

        self.showMessagePage()

    def showMessagePage(self): 
        self.tableWidget = QTableWidget()   
        ... 
        ... 

        self.layout = QVBoxLayout()
        self.mainMenu()
        self.layout.addWidget(self.tableWidget)

        w = QWidget()
        w.setLayout(self.layout)
        self.setCentralWidget(w)


    def mainMenu(self):
            # create menu
            menubar = QMenuBar()
            self.layout.addWidget(menubar)

            messageM = menubar.addMenu("&Messages") # create a file menu
            newM = QAction("&New", self) # create a QAction
            newM.setShortcut("Ctrl+N") # set a shortcut for it
            messageM.addAction(newM) # and add it to our menu
            newM.triggered.connect(lambda: self.showMessagePage())

            contactM = menubar.addMenu("&Contacts") # create a file menu
            importM = QAction("&Import Contact", self) # create a QAction
            importM.setShortcut("Ctrl+I") # set a shortcut for it
            contactM.addAction(importM) # and add it to our menu
            importM.triggered.connect(lambda: self.getExcel()) 
            
            listM = QAction("&All Contacts", self) # create a QAction
            listM.setShortcut("Ctrl+A") # set a shortcut for it
            contactM.addAction(listM) # and add it to our menu
            listM.triggered.connect(lambda: self.showListingPage())