I installed Lubuntu on a friend's old Dell Inspiron 1420, which has a Broadcom BCM5906M wireless card. It worked fine for a few days, then we set up a connection using his Nokia as a modem, and after that the office wireless network didn't show up. Sometimes a network from down the road was listed, but not the office network.

Sometimes a message along the lines of 'your system has encountered a problem' showed, even after rebooting a couple times, but everything else seemed to work fine.

I tried setting it up manually using Edit Connections, but it didn't find the network.

Oddly, Windows 7 on the same computer had a similar problem (after it installed a lot of updates, which may or may not be relevant): the network didn't show; sometimes the neighbour's network did; but connections via the phone worked fine. That makes it look like a router/network issue, but all the other computers using the office network can connect as normal.

Which makes it look like a hardware issue on the Dell. There is what seems to be a wifi switch on the front-left of the computer but nothing seems to happen, either in Windows or Lubuntu, when it is slid across, and the LED light next to it doesn't go on or off. We didn't knowingly touch it anyway between yesterday, when all was working well, and this morning. It also wouldn't explain why other networks would show and not the office one.

I tried to install another driver, as indicated in the post below, but it just said something like 'cannot find a compatible wireless card', so I guess that's not the issue.
http://ubuntuforums.org/showthread.php?t=2090138

So I'm a bit confused.