the demo doesn't even start up...

[unclejunky]

i got the demo on steam because i wanted to try it and whenever i click play demo it say's the files weren't synced with steam. nothing wrong with that. so i click okay and it doesn't do a thing no black screen, no crash, nothing. same thing with the exe in the game files. i dont know what is happening. can anyone help?

[wibou]

I had the same issue a while ago with a different game than KSP, I believe it is a Steam issue when it sometime corrupts its database. You might want to contact Steam support about it.

Are your other games still work properly and/or are you able to install/uninstall other games?

You might want to read of those links:

Disclaimer: be very careful about those as it could destroy everything

https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638

http://www.howtogeek.com/116225/6-steam-troubleshooting-tips/

[vexx32]

In addition to the above... you may want to simply try having Steam verify that the KSP Demo was downloaded correctly by opening up the Properties dialog from the right-click context menu on KSP. From there, go to the Local Files tab and select Verify Integrity of Game Cache.