Windows 7 PC was in workgroup and it went away

I had created a network with my XP machines. I'd shared files and the network was working both ways. Today it just disappeared in the middle of the day. The other machines can't see the 7 machine. It can't see the XP machines. The Networks still appears in the control panel and troubleshooting finds no problems.

Update. Now the 7 machine shows and shares on XP machines, but the 7 machine can't see the XP machines.

Thanks.

My network reappeared. No setting changes. Nothing. This is almost as frustrating as having it go away.

I'm sure "I'll be back".