How closely did you follow the Migration / Installation instructions for DSM 7?
Make sure the user PlexMediaServer has permissions to access the media directories.
DSM 7 - Plex Media Server - Release Info & Migration instructions
DSM 7 - Packaging 2.0 (PMS 1.24.2.4973) - Release information
Synology FAQ - Questions, Answers, and a few How-To’s
Log files always help:
Reporting Issues with Plex Media Server
