I had a similar problem. My 64-bit desktop wired into the network and my 32-bit laptop wirelessly connected, stopped recognising one another on the homegroup. Recreating the homegroup and rebooting didn't help.
Then, when I clicked "See full map" in the Network and Sharing Center, I discovered that even though I had set up my network as a Home network and it was displayed as such in the Network and Sharing Center, for some reason Windows 7 didn't "see" that. It wouldn't display the network map because it said I was on a Public network.
So, I changed my network to a Work network, then back to a Home network, and the homegroup started working again. I had deleted the homegroup on the laptop prior to making these changes, so I simply...