Changing your password unclaims the server. You need to re-claim it.
If you’re on the server, go to http://127.0.0.1:32400/web. Then go to Settings → Server_Name → General and claim the server.
If you are on a different system but on the same network, use the IP address of the server. For example, if the IP address is 10.1.2.3, use http://10.1.2.3:32400/web.
Claiming the server registers it with your account at plex.tv. Once it is claimed, you should be able to access it via https://app.plex.tv/desktop.
EDIT: If the above does not work, see the following:
You need to reclaim your server, but don’t dare to touch the Windows Registry
