What to do with "VAC was unable to verify your session" error:

https://support.steampowered.com/kb_article.php?ref=2117-ilzv-2837

1- Close Steam
2- Windows Key+R
3- Enter C:\\Program Files (x86)\\Steam\\bin\\SteamService.exe /repair (If Steam is installed in another place replace C:\\Program Files (x86)\\Steam with your path)

4- Wait for it to close it self
5- Search cmd in your start menu, right click and start as Administrator
6- Enter bcdedit /deletevalue nointegritychecks
7- Enter bcdedit /deletevalue loadoptions
8- Enter bcdedit /debug off
9- Enter bcdedit /deletevalue nx

10- Close the cmd you opened
11- Restart Computer
12- Start Steam
13- Test if CSGO works again

Additional conflict could be CCleaner. Cheers.

More simple things that could solve it sometimes:

* Go offline on Steam, log out, exit and login
* Run Steam as admin
* Verify files
* And the classic Restart pc