I have seen this several times on different installations. Fileservers stops responding to requests against shares on that server.
Sometimes users with roaming profiles stored on that server gets a temporary profile.
And they cannot connect to network shares on that fileserver.
A reboot of the server helps.
I cannot see any cpu, memory, disk performance issues when this happen, but I can be wrong on this.
What can I do next time except rebooting the server? Can I check some service or what?