I have been using Ubuntu 8.10 since it was released. After I ran update my computer lost contact with the wireless card. This was probably the first update since the upgrade.
The wireless card is Dlink DWL-G520.
$ ifconfig
eth0 Link encap:Ethernet
[...]
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:3452 errors:0 dropped:0 overruns:0 frame:0
TX packets:3533 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:2452784 (2.4 MB) TX bytes:708283 (708.2 KB)
Interrupt:23 Base address:0xc000

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:309 errors:0 dropped:0 overruns:0 frame:0
TX packets:309 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:107989 (107.9 KB) TX bytes:107989 (107.9 KB)

Running a program called "Hardware testing" from the System menu in Gnome shows the following for network controllers:
Atheros Communications Inc. Atheros AR5001X+ Wireless Network Adapter (rev 01)
VIA Technologies, Inc. VT6102 [Rhine-II] (rev 74)
Trying to bring it up manually gives no result:
$ sudo ifup ath0
ath0: ERROR while getting interface flags: No such device
Internet Systems Consortium DHCP Client V3.1.1
Copyright 2004-2008 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/
SIOCSIFADDR: No such device
ath0: ERROR while getting interface flags: No such device
ath0: ERROR while getting interface flags: No such device
Bind socket to interface: No such device
Failed to bring up ath0.

Any help would be appreciated. I am a user at average level.

The 1st thing to check is which network interface is selected for use. Left click the Network Management icon in the applet bar and it will show use and give an opportunity to change. The next place I'd look is at the driver. Did you compile the driver yourself? If so you need to recompile whenevel you do a kernel update. I've no experience with Atheros so I can't be more help. I'd search this forum with you card model name and perhaps Atheros to collect some research. When I have problems with Ubuntu I also find http://ubuntuforums.org helpful.

Thanks for your answer!
Left click on the Network Manager shows "Wired network device is unmanaged" and it is grayed out. There is also a VPN menu alternative.

Right click gives a menu:
"Enable Networking" is checked.
Choosing "Connection information" says:
Error displaying connection information:
No valid active connections found!

Choosing "Edit connections" opens a dialog:
Tab "Wired" is empty
Tab "Wireless" shows the text "Auto" followed by the name of my network
Choosing "Edit" shows various information of which I am capable to understand "Connect automatically".

The wired communication works.

Quote: "The next place I'd look is at the driver."
Where can I find the driver information and exactly what means "I'd look at" in this context?

Forgot to say: I did not compile any driver myself (at least not intentionally)
Searching the archive did not give any ideas.
I tried:
lsmod |grep ath
Gives nothing.

I have no familiarity with Atheros. Some WiFi chip sets are well supported and just work. For some chip sets a package called ndiswrapper is used to run a Windows driver/DLL. I went to http://ubuntuforums.org/ and did a search on Atheros which got 250 hits in those forums.