Failed to create D3D device! error while opening Source games

Garry’s Mod and all other Source games have been working very well on my PC, but suddenly, they crash on start up. I exited GMod earlier today, and when I started it again today, I get the Failed to create D3D device! error. I reinstalled all my drivers and DirectX software. I can start the game by setting -dxlevel 80 in the launch options, but when I change my resolution to widescreen, the game crashes again. Any clue how to fix this?

• Nvidia 9800M GS Graphics Card
• Intel Core 2 Duo Processor
• 4GB of RAM
• DirectX10

Edit: Fixed. For some reason it completely fails to run on 1366x768, my preferred resolution…

Video card don’t have power to make Big rezoliution max for Source game 1200x1000