Re: Wifi stops working after sleep mode or hibernate.
I just went through this with a desktop when I replaced the wireless card. For whatever reason, the router wasn't renewing the connection when Windows resumed. Given that I have no issues with the laptop used in the same network, I suspect it's a firmware issue in the adapter card. In any case, I solved the problem by assigning a static IP to the machine.
You can use IPCONFIG to determine the IP assigned to the laptop and the IP for the gateway. You'll have to do some research though, to get the DNS server addresses for your ISP.