*Since this issue occurred after a configuration change on a Layer 3 switch, the most likely cause is misconfigured ACLs (Access Control Lists).
*ACLs control which traffic is allowed or denied, so an incorrect ACL may be blocking access to the file server.
*Why not the other options?
*Switching loop (B): A switching loop occurs at Layer 2 (not Layer 3) and causes network-wide broadcast storms, not just loss of access to a file server.
*Duplicate IP addresses (C): This would cause connectivity issues for the devices with the conflict, but not necessarily prevent all users from accessing the file server.
*Wrong default route (D): The default route is used for traffic leaving the local network. If users are unable to access an internal file server, this is unlikely to be the issue.
Reference:
CompTIA Network+ (N10-009) Official Guide - Chapter 8: Network Access Control and ACLs