After the password change you must re-claim your server (i.e. connect it back to your plex.tv user account.) Only then it can be used by all your plex clients.
Start by clicking on “Open Plex” in the context menu of the Plex task tray icon.
Then enter Settings - [server name] - General and click on the Claim button.
If there is no claim button, you may have to resort to this method: Unable to access server after password reset - #116 by OttoKerner
