Could be a lot of things. Does it run at all? If it won't actually start, might be the fuel pump. Turn the key to the run position without trying to start it. You should be able to hear the pump running for a couple of seconds. If you can hear it its fine, if not then do some more trouble shooting in that direction. If it runs but won't accelerate, have the codes read to find out what the computer is telling you.
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