I loaded an old project of mine in 2018.3.12f1, clicked on Play and got a bunch of errors related to a package that was installed in my project. Since this package was not used in my project, I decided to deinstall it but, oh horror! the package manager link in the Window menu was missing!!! After a while wondering what I should do, I decided to investigate the packages that were present in my project. I right-clicked on the one that generated errors and clicked on "Show in Explorer". I quit the editor and deleted all the packages that were in the window that opened (LoadedProject\Library\PackageCache). After starting the project again the deleted packages were reinstalled. Doing that not only fixed the errors in play mode but the Package Manager was back in the Window menu! I hope this will be helpful if you have that specific problem.