There are many things that can go wrong with every wireless network, here are are a few tricks and tips you may want to refer to if you start having a problem with a wireless networks that worked just fine a moment ago.
Some of these tips may seem somewhat obvious, but you should check anyways - I know I have spent hours looking at the more difficult possibilities when the solution was as simple as restarting the computer.
As is the case with many computer problems, restarting the computer can be very helpful. This is especially true if the wireless connection went out because of something like interference from a cordless phone call.
Make sure that you have connected to wireless network that you intended to. Oftentimes, Windows will automatically connect to a network outside your house, especially if the other network is very close.
Things like wireless (cordless) phones and large metal objects can mess up your wireless signal. Make sure that you have a pretty clear path to the access point.
Sometimes Windows, for no apparent reason at all, drops the security code you entered when you set the wireless network up. Read the documentation included with the wireless router and your wireless network card and find out how to set that stuff up - it's quite different on different systems.
I often find that restarting the router (try unplugging it and plugging it back in) sets everything on the right path. If you start pulling your hair out and cursing because the confounded thing is just not working - try restarting the router.
If you still can't figure out why the wireless network is giving you trouble - ask a question in our forums. Please include changes you have made recently and potential reasons why you could be having these troubles.