When a Windows 7 PC resumes from sleep or hibernation, USB device problems are not uncommon. Windows 7 is really bad at restoring the connections as it wakes much worse than Windows XP Pro ever was. When this occurs you will see error messages, USB devices not working, USB devices not being visible, and/or USB devices not working as expected. (Experienced with Windows 7 Enterprise 64-bit Enterprise SP1).
Why this problem occurs
This problem occurs because the Plug and Play operation that is performed when the computer resumes from sleep or hibernation does not correctly recognize the USB devices or power saving settings are enabled. When the computer is connected to many USB devices and resumes from sleep or from hibernation, multiple and concurrent Plug and Play processes are required. In this scenario, sometimes the Plug and Play operation cannot recognize all the USB devices that are connected to the Windows 7 or Windows Vista-based portable computer. Therefore, the USB devices may not function correctly. 
You should not need to scan for hardware changes every time that the Windows 7 PC wakes from going to sleep, this seems to be a pretty fundamental flaw in the Win7 plug-and-play operational model.
Sadly, Windows 7 sleep problems are all too common. By knowing where to find the numerous sleep and hibernation settings, you can determine how to troubleshoot the commonest symptoms. 
Fixing the problem:
* The “USB selective suspends settings” option in the Power Options may be Enabled. (Power Options) switch it to Disabled
* The “Allow the computer to turn off this device to save power” setting in Device Manager may be Enabled. (Device manager) remove the tick
 How to troubleshoot USB device problems that may occur after a PC that is running Windows 7 or Windows Vista resume from sleep or from hibernation KB953367 [MS]
 Resolving Problems with Sleep and Hibernate Modes (Windows 7) [HP]
 Troubleshooting Windows 7 Sleep Problems [Computer Performance]