Take it you are talking about a Wireless connection. Take it you have tried the obvious and rebooted the router.
Maybe try Device Manager and have a look to see what wireless driver it is using. If it is a Microsoft driver click on update driver. and click on browse my computer for driver software.Then choose Let me choose from a list of devices.
Choose the one that is from your wireless card and ignore the Microsoft update. It may be an older driver but install it anyway.
I had the same problem on Friday and that is what fixed my connection problem. it may not yours but worth a try.
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