before you do something drastic like reinstalling windows ........get the windows updates first and reboot PC after finished
update your video card drivers & any other updates you need
in steam check the games go to SSE / property / local files and click verifies game cash integrity ........this will download any missing files that may have been removed or corrupted
make sure you have the latest Bethesda updates
when doing any editing on Skyrim.ini or Skyrim.pref its strongly recommended to back up both files just in case you have issues like these
there is a small app called Beth.ini that will do this & you can configer heaps of settings with out you needing to copy and paste codes
if all of the above fail
go to your documents /my game /SSE and cut and paste both files (ini & prefs) to your desktop) ......now run the game with out the files, you should get the Skyrim options window & it will detect your card again, if you go and look at the folder where you removed the files you will see two NEW files created (default)
now after doing all of the above steps your game should work
anything you do always back up the files first ........one wrong code in the wrong place, a wrong number can stuff up your game