On your way to becoming a shortcut ninja you may have chosen to hide the menu bar in your Signal Desktop preferences.
To view the menu again, open Signal Desktop and press Alt+F to bring up the File menu and access Signal's preferences so you can re-enable the menu bar.
Here are more shortcuts:
|View all shortcuts||⌘ /||Ctrl /|
|View Signal Desktop preferences||⌘ ,||Ctrl ,|
|View the menu again||Alt F|