Question : Network workgroup - "Work group is not accessible"

I set up a workgroup using Microsoft network and it all worked fine until recently. Operating systems are XP and XP home editions. I had one computer on the system that would not work properly and periodically connected with the other computers and the server but always connected to the internet. That computer had not been used until recently. When it was turned back on it didn't connect with anything except the internet.  Now other computers on the network - all named workgroup - have stopped seeing each other - and doing so at random. Some computers see some of the computers on the network - and some do not.
I also have a server on the network that is visible on all computers. I also have a G5 - Mac - using OS 10 and it still sees and connects with everything. Since then I have turned off the computer that didn't work at all have this problem. This is message I most commonly get when I click on workgroup to display the network…

"Work group is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions.

The list of servers for this workgroup is not currently available."

I also connected a laptop today that had not been on the network since this problems began. When it first came on it saw everything. When I clicked on a link to another computer all the icons disappeared and I got the same message.

Please advise.

Answer : Network workgroup - "Work group is not accessible"

OK, to close this out, click on the accept button to the right of the comment that helped, and give it a grade
Thanks