The shortcut 'Ctrl+M' should be considered for removal as it will hide the menu bar and is no way to bring back the menu bar via mouse click. Most people will know 'Ctrl + N' and may accidentally hit 'Ctrl + M' without knowing it. In my opinion, shortcut keys should only be assigned to frequently used functions. I think that hiding the menu bar functionality should exist, but maybe not as a default shortcut. The more shortcut keys assigned by default the easier it is to accidentally hit one and in this particular case there is no way to restore the previous setting without knowing the exact hotkey combination. Two solutions exist, remove the default shortcut key or provide the user a way via the mouse to restore the menu bar. I get that this may be pedantic but I lost the menu bar and had no idea how to get it back. Had to search for a while until I figured out what was going on. I just don't want another newbie user accidentally hiding stuff in Dolphin because they wanted to open a new window in the containing folder 'Ctrl+N'.... but finger salad 'Ctrl+M'
There is a graphical way to bring it back via the mouse: click the hamburger menu > For X more actions > Show full menubar. Since you had the menubar on but it is off by default, you must have known how to enable it in the past--either by using the keyboard shortcut or the aforementioned method in the hamburger menu. Therefore it is reasonable to assume that you would remember one of these methods and use it to turn the menubar back on. There's only so much we can do for the user. :)
I don't see the hamburger icon in Dolphin. I have edited the toolbar, but it's doesn't show in this screenshot either https://apps.kde.org/dolphin/ If there is a hamburger icon in a newer version that allows the user to switch it back on that's great. But I don't think that the menu bar is off by default, again shown in the screenshot above. However I still think my point about shortcuts still stands. It shouldn't be default and it's an easy key-combo to accidentally strike. The solution is super simple too. If there is someone who requires that specific functionality they can assign it. I've given my experience as a user, I would have appreciated if someone had thought of this edge case beforehand. It would have saved me a good chunk of time figuring out how to undo it.
Maybe is this what Dolphin looks like by default? https://ubunlog.com/wp-content/uploads/2020/05/KDE-plasma-5.18.4-y-Dolphin-1024x576.jpg
Ah I think I screwed myself here, I most likely got rid of the hamburger icon. I agree at that stage there isn't much you can do for the user when they throw the paddles out of the boat. Shortcuts only for frequently used tools though still stands! xD