When you say "I can connect to the NAS" and "my shared folder is gone" what do you mean? Various interpretations of these are...
Connecting to the NAS:
DSM web GUI
DSM File Station
Windows file sharing
Third-party application on Windows or Android (I assume not Mac or iOS from your profile info)
Shared folder is gone:
It's not available within DSM Control Panel's Shared Folder?
It's not available with DSM File Station?
It's not listed or responding when creating a mounted network share in Windows?
It's not mounting as an already created network share in Windows, when Windows was set to auto-mount it?
It's not listed in the available shares when using a third-party app.
There are so many ways to connect to the NAS. I don't have any auto-mount to NAS shares, using Syno Drive instead. I mount as needed using whichever is the best/available service SFTP, SMB, AFP, or WebDAV. So more information about how you use your NAS is best otherwise people write back very long responses to very short calls for help