You probably disabled the service... I can't tell you how many people take blackviper's advice too far...
http://support.microsoft.com/default.aspx?scid=kb;en-us;311504No Items Appear in the Device Manager List When You Open It
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This article was previously published under Q311504
SYMPTOMS
When you open Device Manager, no devices appear in the list. Instead, a blank window appears.
CAUSE
This behavior occurs if the Plug and Play service is turned off (disabled).
RESOLUTION
To resolve this behavior, set the startup type for Plug and Play to Automatic. To do so, follow these steps:
Click Start, click Run, type services.msc, and then click OK.
Double-click Plug and Play.
If you receive a Configuration Manager message, click OK.
In the Startup Type list, click Automatic, and then click OK.
Close Services.
Restart the computer.
STATUS
This behavior is by design.
MORE INFORMATION
Plug and Play is a core operating system service, and therefore, it is best to not disable it. It runs in normal mode and in safe mode.