Although this might or night not apply to you or your situation ... I had the same problem running Windows 7 Home premium 64bit and after my Gadget gallery was working fine, suddenly it vanished and nothing would make it open!
I searched the web high and low, and like most questions there were litterally hundreds of answers and solutions, AND NONE SEEMED TO WORK (for me at least)!
In my case I got lucky, as the cause of my problem turned out to be a 32bit Gadget that I'd downloaded and was running, and as a result it changed my gallery to 32bit mode, which wouldn't disply in my 64bit Windows version!
My "luck" was downloading ANOTHER gadget off the Microsoft website gallery called: "64 to 32 Sidbare" !
As soon as I installed it, the gadget returned my autorun 32 bit sidebar back to 64 bit, and ((((POOF)))) my gadget gallery suddenly reappeared and EVERYTHING was back to normal!
A bit of an unorthodox approach I guess, but it woked for me, and if you are runing 64 bit windows, and all else fails, you might want to try this simple fix!
The Gadget "64 to 32 Sidbar" is located in the on-line windows gadget gallery under Miscellaneous (or at least that's how "I" stumbled across it)!
I hope this helps !!!!
