Simple file sharing hosed?

I setup a network about 8 weeks ago and have one system designated as he
server. All the systems are freshly imaged. I created a share using simple
file sharing to keep all the companies data in one place for easy backup. All
was well until about 2 weeks ago. Now no one could get to the share. Their
network mapping are gone and they are prompted to authenticate to access the
share. The authentication has the guest account in the username box and will
not allow access to the share. Any ideas as to what happened. I have since
created accounts on the server for each user which allows them to get on with
business but would like to know what happened. Any help out there?