Try to go back to some restore point. Start with a recent one, if it don't work, try the previous one untill you try the oldest one.
If any restore point work, congratulation, you're done. Stop there.
If the previous don't work.
Using another computer, download some Windows 7 DVD image from some torrent site. Burn that image to a DVD-R using your favorite burning application.
Doing that to replace a lost genuine installation DVD is legaly acceptable if not totaly "legal". If you find one branded to your actual computer brand, all the beter.
When asked what kind of install to perform, chose "Update".
This option will preserve all your files, installed applications and settings.
Next, let Windows updates do it's job reapplying all needed updates.
If you don't get the update option, all of your actual installation will be found in a folder named "Windows.old".
Electro, September 2013