If your Main Menu toolbar is hidden you can get it back by pressing Ctrl + Shift + N (or double tap Shift on Android Studio 4.x) and search for the View | Appearance: Main Menu Action. Toggle to ON to enable the menu again:
If your Main Menu toolbar is hidden you can get it back by pressing Ctrl + Shift + N (or double tap Shift on Android Studio 4.x) and search for the View | Appearance: Main Menu Action. Toggle to ON to enable the menu again: