KSP doesn't start anymore

[Fox]

I haven't launched the game for a VERY long time and suddenly realized that it completely stopped working. I reinstalled the "clean" version, but the result is still the same. No result. The game freezes at the loading stage, and when trying to launch from the 64 shortcut, Steam doesn't even "see" that the game is running at all.

[Lisias]

Really weird. I would suggest to completely remove by hand the current KSP directory (and contents), and then install from scratch.

Also be sure you are not using one of that old tricks to bypass the (in)famous PD-Launcher - it was removed, and so not needed anymore.

If even by doing that, the game still don't launches, then I would need to give a peek in your KSP.log to see what's happening... Publish it on a file service like dropbox and paste the link here.