A user can sign in but cannot access a shared folder they used yesterday. How would you isolate the cause?

Confirm the exact UNC path and error, whether other users are affected, and whether the issue follows the user to another device. Verify name resolution, server availability, SMB connectivity, account state, and recent group or ACL changes. Check both share and NTFS permissions and whether stale sessions or credentials are involved. Look for relevant access-denied events under the organization’s logging policy. Do not remove permissions broadly or disable the firewall as a blanket fix. Restore the intended access and record the verified root cause.