If you’ve formatted your C: drive, then you’ve deleted everything what constituted your server. All its libraries and the lists of media therein, and its “identity”. Nobody would be able to actually access it anymore.
After installing Plex media server afresh, you need to “claim” the server – i.e. connect it to your plex.tv user account. Do the installation and the claiming while being logged in to the Windows user which you are using for regular work.
New Claiming Requirement for Windows
Depending on the web browser you are using, verify that you are not running into this issue:
https://forums.plex.tv/t/important-note-about-the-plex-web-app-local-network-access/933264/1
