I realize this thread is a very old zombie, but might as well get a reply out there:
The first question is if you ever changed it from the default. If not, you should (see below) but the default WEP key is almost certainly printed on the label on the bottom of your router.
If you have changed it then you'll need to go to your router's configuration settings. How you do that will depend on your router, but most modern ones let you do so via web browser:
-Log on to a computer that's connected to your WiFi network.
-Bring up your web browser and point it to your router's IP address (usually 192.168.0.1 or 192.168.100.1, if it's not those then check the label on the bottom of your router for the default IP address).-If you've changed its default IP address then there are other ways to find out what it is. If you're using Windows then just pull up the command prompt (Start > Run > cmd) and type "ipconfig". You want the IP address listed for the default gateway of your wireless network adapter
-What you do now will depend on your router, and I'm not familiar with 2Wire's setup. However, you'll probably have to log in. The default username and password should be on that handy label on the bottom of the router. If not, a username of 'admin' and no password, or no username and a password of 'admin' or a username and password of 'admin' are normal defaults for most.-If you don't know and can't find your username or password then you may want to reset your router. Instructions will probably be on 2wire's website. Usually you have to hold the reset button for 5+ seconds. This will erase all of your settings on most routers and set them back to default. If you haven't changed them from default, then that doesn't matter much. If you have, then you'll have to set everything back up.
-If your router still uses the default username and password you should change them immediately.
-Once you've logged in there should, somewhere, be an option to view or set your WEP key. If it doesn't list one then just change it to something you can remember. Just be aware that everything currently connected to your wireless network will lose their connection until you tell them to use the new WEP key.
-However, WEP is very insecure. It is possible to, running only two or three completely free and openly-available programs, break into any wireless network using WEP in seconds. I've done it to my home network as a test a few times. Doesn't matter how strong your key is.-As such, you should, if you have the option, switch to WPA2. It is significantly more secure and effectively hack-proof. Note that some gadgets may not support it (though the only one I can think of is the original and Lite-flavour DSes). There should be an option somewhere to use WPA or WPA2 instead of WEP (probably a drop-down menu or list). If WPA2 isn't an option then WPA is still better, though not perfect. The particular kind of WPA2 doesn't matter (TKIP or AES or whatever).
-Note that switching to WPA2 will, like changing your WEP key, cause everything connected to the wireless network to lose its connection until you connect them again with the WPA2 key.
Hope that helps.
Edit: Abuh? This thread has over 100 000 views. Some of that will be web crawlers, but I have to think that we've had at least a few thousand people Googling this and ending up here. Which is...I don't know. Don't 2Wire products come with manuals? A website? Something?