Why did my car stop accelerating ?

Sound like it was in reduced power mode, or limp mode. That's when the computer detects a significant problem and cuts power to the engine so you won't damage anything while you drive it to a repair shop. They often do reset after shut down, butunless it was just a one time glitch, it will be back. That said, without codes, unless its the code that you've already got, its pretty tough to trouble shoot. For now I would address the trouble code that you have and keep on motoring. Also, do a little research on the code you have, like here: https://chevroletforum.com/forum/cobalt-32/p0446-code-2006-cobalt-36231/ It may be a wiring issue rather than an actual part. As these cars age the wiring breaks down and often sends codes that can be corrected with some simple maintenance.