It sounds like you are experiencing a common issue. You might have spontaneously loss your permissions to view the device manager. In order to fix this, if this has just happened recently, you can do a system restore to a time before this started happening.
Alternatively, you can edit the registry to fix the permissions but this is more complicated and requires a little experience with the registry:
1. Go to Start=>Run=> Type "regedt32" and press enter
2. Expand the "HKEY_LOCAL_MACHINE" main branch ( click on the [+]/arrow sign to the left )
3. Scroll down and expand these in sequential order "SYSTEM\CurrentControlSet\Enum"
4. Now, right-click on the "Enum" folder and select "Permissions"
***If there are currently no permissions then this is indeed the problem and you are on the right track so proceed***
5. With the "Permissions for Enum" window open, click the "Add" button
6. Type in "everyone" and click "Check Names." then press "OK"
7. Click on the "Add" button again, this time repeat the previous step and type "system" instead of everyone
8. Now with "Everyone" and "System" in the "Group or user names" list, select "Everyone" and tick the check box across from "Read" under the "Allow" column, then click "Apply"
9. Select "SYSTEM" and tick the check box across from "Full Control" under the "Allow" column, then click "Apply"
10. Click the "Advanced" button at the bottom of the window
11. On the "Advanced Security Settings for Enum" window, check "Replace permission entries on all child objects with entries shown here that apply to child objects."
12. Click "Apply" then "OK"
13. On the warning window click "Yes"
14. Click "Apply" then "OK" again
15. Close regedt32
16. Restart your computer
Device manager should be working properly now. If it is still showing an empty box, and if you've noticed that the permissions already existed after step 4, then you are experiencing a more complicated problem and I suggest you try a system restore first.