Engine running rough

Hi, I have a question last night some people took my car for a test drive trying to sell it, and it ran fine when they took it but when they brought it back and said it was running rough and it was the timing belt.
I didn't think anything of it then went out later to go to town and started it up and I was running rough while idling and shaking.
Any one know what is wrong with it, could it be the plugs?
thanks