Cannot access computer on home LAN

I have a home lan - with both wired and wireless computers. I suddenly
cannot seem to look at files, etc. on one of the computers. The
computer shows up in windows explorer under my network places, but when
I try to explore the computer I get the message...

\\ComputerName is not accessible. You might not have permission to use
this network

resource. Contact the administrator of this server to find out of you
have

access permissions. The network path was not found.

Then, if I try to use ComputerName to look at the other computers on
the lan, even though I do see them in Network Places, I get the
message...

WorkgroupName is not accessible. You might not have permission to use
this network resource. Contact the administrator of this server to find
out of you have access permissions.
The list of servers for this workgroup is not currently available.

I have no idea what to do!
Please help.
Thanks.
Elias