Can you give us more information - for example:
What WiFi connection are you using - built-in, dongle, or PCMCIA etc?
Have you checked in Control Panel - System - Hardware - Device Manager to see if the WiFi device is present, has drivers loaded and is working?
Can you click Start - Connect To - Wireless Network Connections, and does "View Wireless Networks" bring up anything? If so, what?
What happens if you try to establish a DHCP address that way?
Alternatively, do you have a software firewall installed? Might that have updated and glitched?
That might tell us whether the problem is in the computer or the Wireless Access point.