A network share is a folder on another machine made available over the local network. Reaching it needs three things to line up: a reachable address, an agreed protocol, and accepted credentials.
Name resolution fails first and most often. The share is reachable by its numeric address but not by its name, because local name discovery is unreliable across different device types.
Protocol versions are the modern trap. Older network storage speaks an outdated version of the file-sharing protocol that current systems disable by default for security reasons.
Saved credentials cause the third pattern: a share that worked for months suddenly refuses access after a password change, because the stored login no longer matches.