A few things to try:-
Have you checked that the wireless hasn't accidentally been switched off?
You could try connecting by ethernet and then disconnecting to see if wireless works.
Is your wireless adapter card up to date? Go to Device Manager to update it
Click on Start and then Control Panel. Click on the System and Maintenance link. Note: If you're viewing the Classic View of Control Panel, you won't see this link.
Simply double-click on the Device Manager icon and then proceed to Step 4.
In the System and Maintenance window, click on the Device Manager link near the bottom
of the window.
You can now go to your wireless adapter, right click and choose update.
Have you tried deleting the wireless connection and trying to re-connect?
Have you recently had a windows update? If so you could try to do a system restore.
Navigate to the Start -> All Programs -> Accessories -> System Tools program group. Click on the System Restore program icon and choose a restore point.