What causes a timing belt to jump and oil in spark plugs, how much should it cost?

i have a 1997 honda accord 2.2 4 cylinder. It has 305,000 miles on it. I was driving the other day and it worked just fine, all of a sudden it stopped working and my radio was still playing, i check under the hood and the was oil in my spark plugs, people are telling me my timing belt jumped time, what causes it, and how much should it cost? It acts like it wants to crank but it doesnt

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