Trying to recreate this one but it happens a lot when I am working on a project currently without a concrete cause. Sometimes the Project settings window disappears then any attempts to open it after by going through the Edit menu or Build Settings > Player settings just doesn’t do anything.
Edit: saving the window layout for the editor then restoring it or changing does open the Project settings menu but I still don’t know why it disappears in the first place.
This is a known issue and will be fixed in a11.
…until that fix is out, if you look for a tiny “slash” of grey with a drop-shadow on your screen, that’s the Project Settings window 
a11 is out today.
But until then: Unity Editor Preferences not opening
If anyone else encounters this issue you can simply “Revert to factory settings” your layout and the window should appear as normal. It worked for me at least in Unity 2020
Update:
You can also try selecting one of the default layouts if you don’t want to revert to factory settings. Changing the layout should reset the size of Project settings window as well.
Thank you! That fixed it for me. Was having this issue in 2020.1.9 and 2020.1.10.
When the layout reverted to default it also threw this error in the console
Failed to destroy editor windows: #1
ProjectSettingsWindow
UnityEditor.<>c:<ReloadWindowLayoutMenu>b__35_0() (at /Users/bokken/buildslave/unity/build/Editor/Mono/GUI/WindowLayout.cs:507)
I have to routinely delete ProjectSettings to get the Services menu to pop down