Your post is void of the wifi details but this issue is usually about settings like a hidden SSID or use of the WEP or lesser security. Changing RF channels can help too.
Then again, unless I take the machine that has lost connection to the room that has the router I can't test if that machine has an issue.
Sorry but this does not sound unique. Just another wifi setup or hardware issue.
I am having what I think is a unique problem. I have a WRT54GS v7 that has been working wirelessly for a while. The router is downstairs and I have 2 desktop computers upstairs which connect wirelessly and have been working fine since the network was established.
Last week, I bought my daughter a new laptop, but I couldn't remember the password (the file for the network configuration was on another computer and somehow that was misplaced as well). Cisco was very helpful helping me restore the network, but once I did that, the network is no longer showing up on the computer upstairs. That computer uses a WUSB54GS v2 wireless hub.
Now, I know the signal can be seen upstairs as that is where my daughter works. Also, other networks can be seen by the desktop, but not mine. Does anyone have any idea why that computer can see other networks, but not mine?