for me, this problem occurs if the navigation bar disappeared in one file and it still exists in other files. my decision: 1) to find the project in which the missing file of the navigation bar is located; 2) unload the project (in the explorer-> solution, right-click the name-> project in the pop-up dialog box, select unload the project). Logically then the file will be closed, and the project will be gray; 3) load the project again (in the explorer-> solution, right-click the project name-> in the pop-up dialog box, select load the project). Then the navigation bar will reappear in this file. This solution also helps for Intelisense not working for some files.
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