Sorry that the above method did not fix! The other cause for a Blank Device Manager is a corrupted key in the Registry. Before i detail the steps to rectify (Thanks to Microsoft Help), kindly carefully read and follow this tutorial of SevenForums: Default File Type Associations - Restore
The above tutorial will help you IN CASE the steps outlined below are not followed to the letter. It helps you to restore the registry!
Now, to the actual steps;
1. Click Start-> Run->type in regedit.exe
2. Locate the following: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum
3. Now right-click on Enum
and choose Permissions
4. Click the Add button and type in Everyone
and click OK. Check the Allow box next to Read for Everyone.
Now click Add again and type in System
Check the Allow box next to Read and Full Control for System. It should look like this:
5. Both the check boxes under Allow should be selected when you highlight System.
Also be sure to click on the Advanced button after adding both users and selecting Reset permission entries on all child objects with entries shown here that apply to child objects.
Restart your computer now and check if the issue persists.