Hey prem lol i also found myself in the same situation. I came here for a solution but didn’t find any useful suggestion.. so i started tnkering around and figured out the issue..
See when you pressed
Windows> customisation> menus> User menue 1
While in startup mode. You basically set the startup mode/layer to be treated as ‘User menue 1’ . Somthing like a reroute or redirect.. so every time you press on the startup it will show you the menues that are in ‘User menue 1’.
The issue is that the layouts from ‘User menue 1’ until ‘user menue 5’ dont have the . ‘Windows ‘ bar to allow you to reverse what you did in the first place. Atleast not easily.
The solution.
Go to >customization >customize menus
And there in the bar you should see ‘M_EDITOR_USER1’ or whichever ‘user menu no’ you rerouted your startup to.. mine was ‘M_EDITOR_USER2’ since i accidentally rerouted mine to ‘user menue 2’
Anyways … under that menue you’ll see 2 submenus.. which is ‘submenu File’ and ‘submenus customisation ‘..
Just switch to a different M_editer menu which has alot of submenues. Like ‘M_EDITOR’ or ‘M_EDITOR_BP’..
Yo dont really need all of them. U just need the ‘Windows ‘ submenu. So just click on windows. At the bottom where the ‘details’ panel is youl see the options to copy.. click copy.
Go back to ur ‘M_EDITOR_USER1’ menue. Or whichever no. U rerouted to. Clich on any of the 2 submenus to make the ‘details’ panel to light up… click on paste. Than ‘apply’ and than ‘save all changes ‘..
Go back and you should see an editional ‘Window’ bar in your top menus.
Now press window >customization>menus> Cinema 4d menu
To reroute it back to how it used to be..
It should work just fine now,👌🏻✨