Driving it and the rpm dropped. Will turn over but wont crank. What's wrong?

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I was driving it. Bought the car used had 150,000 miles on it. Had just changed the oil and filter today because the oil was black and it was smoking out the tailpipe. I was driving it to get gas and all of a sudden the rpm dropped and i would press on the has but it wouldn't go. It died. I pulled it over and tried to crank it. It cranked but was sputtering and then died. I know its not the battery because all the lights and everything come on. Any suggestions on what's wrong with it?