Car wont start...just turns over?

i have a 1989 pontiac tempst witha 2.8L V6 fuel injected..it started but it wont start again..it turns over but wont fire.. i took 1 spark plug out and its covered in black and tried to clean it and it wont come clean. i need help! why are the plugs black? and it sounds rough the engine. when i started it, the motor sounded rough.. there NGK Platinum and were $90 for 6 of them 3 years ago when i bought them and it hardly gets driven ..you can email me anytime at mgleboff@hotmail.com or here