Games not mounting!

Hello there.
I discovered that CS:S isn’t mounting to my Gmod. And yes, I do own CS:S , I have it installed and I have not pirated Gmod. The mounting problem hasn’t happened before.
But not only CS:S doesen’t mount, every game (HL2,HL2:E2 and so on) except TF2 doesen’t mount to Gmod. Mounting box only has that red “x” after the game. What can I do about this, or can I do something?

EDIT: It seems this was because of my computer, and restarting my Com fixed the problem.

I have this problem too. I can only fix it by verifying the games I want mounted and then verifying gmod to get them to work. Wat do.