Firstly, in order to fix the problem, check if your computer meets the minimum system requirements to run Counter-Strike Source:
- CPU Speed: 1.7 GHz;
- RAM: 512 MB;
- OS: Windows 7 (32/64-bit)/Vista/XP;
- Video Card: DirectX 8.1 level Graphics Card;
- Free Disk Space: 4.6 GB.
If the requirements are fulfilled, update your video card drivers and install the latest version of DirectX. Also, press Alt + Ctrl + Delete key combination, open Task Manager and from the Processes tab close all unnecessary applications that are running in background. If the issues persist, uninstall and then install the game again.