That's extremely vague and we won't be able to help you much based on that alone. At this point you need to get a scanner and pull the DTCs (Diagnostic Trouble Codes) from the ECM (Engine Control Module) to get an idea of where to begin diagnosing the vehicle. It could be something as simple as leaving your gas cap loose, or something as damaging as head gasket, we just don't know based on the available information.
I would refer to this thread to have a better understanding of what I am talking about.