Changing your password unclaims the server. You just need to reclaim it.
Point a web browser at the IP address of the NAS. See if you can login & claim the server. For example, if the IP address is 10.1.2.3, use http://10.1.2.3:32400/web.
If that does not work: FAQ 31: Reset / Reclaim PMS on DSM 7 (Not Authorized - Locked Out).
