I have noticed a problem on my Windows 8.1 Preview desktop; my Realtek network card will not connect to the network after returning from sleep mode. The only way to fix it is to reboot the machine.
Other people are reporting that the problem is caused by Virtualbox. I will uninstall it when I get home and try it. If that is the problem, I’ll switch to VMWare Player until Virtualbox gets updated.
Update: I converted my Virtualbox VM to VMWare Player (vmdk format) and uninstalled Virtualbox. After rebooting, I put the machine to sleep and after a time woke it up. I had no problems with the network after resuming. So I think it’s safe to say the problem was with Virtualbox.