I’ve seen enough samples and logs of long-time users to take that conclusion with a ton of salt. 
Does the Plex icon appear in the task tray area? If it doesn’t, PMS could be running under a different Windows user account.
And drive letters for network shares are stored in the user account.
If you stop PMS, log out of Windows, reboot the machine, then first log into the user account that you are regularly using, are the drive letters back?
Maybe you want to switch to UNC paths instead. These are a requirement anyway if you are running PMS as a Windows service.
