USB adapters are notorious for having reception problems. Does it not work in areas of your house/office that it used to work?
I would try the following troubleshooting steps to try and get to the bottom of it:
-Download a program called Netstumbler from here:
http://www.netstumbler.com/
When run, it should give you an idea of Signal Strength to your access point. Monitor these in the places you usually use your computer, and report back on them here for further info.
If it's reception problems, try a booster or some sort of gain using an antenna for more signal strength.
-Are you using the built in Windows Wireless networking client, or the manufacturer supplied one, eg. Intel. Whatever you're using, try an alternative one if possible.
-Check the manufactuers website for an updated driver, failing that try searching for an older driver than you're currently using and try both.
Try the wireless adapter in another computer to make sur the unit itself works as expected.