I ran into the same problem. This did the trick, for me:
Make sure there is no cura.exe process running, by looking in Task Manager (or just rebooting to be sure).
Go into .../AppData/Roaming/cura/4.4 (or whatever version you are running)
Delete cura.lock, if it is there. If it is not there, and Cura is not running, then this isn't the problem for you.
There is no risk, at all, in deleting that file. It gets created at runtime and is really only valid while Cura is running. If it doesn't get deleted when Cura exits, it will cause the package manager to hang.
Hope this helps.
