Sounds like some app you recently installed may have bumped your "Sidebar" from normal operation. Try going to the sidebar, right-clicking, and closing the sidebar. Restart your computer and see if it comes back. I mean specifically the "Add Gadget" window. If it doesn't you may have some Windows file corruption. Could also be due to a new Virus--make sure you do full Virus scans with your main Anti-Virus product (Norton, McAfee, Avast, etc.). You have a couple of choices.
#1: Go to an elevated Admin prompt and run "sfc /scannow" and let Windows try to repair the missing or corrupted files that operate that part of the sidebar/gadget app for you. You may need to feed your Windows7 disc that came with your computer into the DVD drive if it asks you to do so. *MAKE SURE YOU SET A RESTORE POINT AND BACKUP ALL PERSONAL DATA TO AN EXTERNAL HARD DRIVE, FLASH DRIVE, OR CD/DVD DISCS PRIOR TO RUNNING THIS PROCEDURE OR #2".
#2: If you have Restore Points enabled on you Win7 system, go back to the last Restore Point date just prior to the date when you last remembered your Sidebar/Gadgets app properly working; i.e.: a week ago, a month ago, 6 months ago. Again, this is where it's critical you backup all your data, because you will probably lose some stuff doing a System Restore, that almost always happens, so you need to have an externally saved copy to be able to put things back that "Mysteriously" disappear after you begin using your system after the Restore procedure. [You may have done this before, and therefore you know what I'm speaking of. ]
Finally, if neither Option #1 or #2 does the trick you could be fighting a hardware problem that just recently occurred on your system and you are trying to repair it via using a software solution. That will never fix a hardware problem! If this does occur, and I really hope it doesn't since you are running a Win7 computer, it's at least 2 yrs. old or older and it's not beyond the realm of possibility your are experiencing a faulty or failing hard drive or Motherboard or even RAM. It happens all the time and I see it every day. If you reach this point in the process, I suggest you run full hardware diagnostics on your Computer to eliminate those problems. Even so, that's not 100%, so you may need to either replace items *the hard drive is #1 on my list* or take it to a local Computer Pro and have him run the diagnostics for you (for a small fee).
Post back your Make/Model info. and keep us posted on the results of the above procedures.
Good Luck!
BIGBEARJEDI