Answer
Fuel filter. spark plugs. ignition wires. battery terminals not on tight (even if they appear to be tight). water in the fuel tank.
bad o2 sensor.
those are probably the easiest and most common trouble areas in starting a car, also the cheapest to fix (the o2 sensor would be the most costly of all of them .. roughly $30-$40).
might be worth the few dollars to check/change each one of them and if not, you may need to take her into a mechanic to let them check it out.
you could try to run codes on the engine to see what it tells you. it might pop back and tell you the o2 sensor is bad or it may give you another "code" to give you some insight as to what is wrong.
http://www.allpar.com/fix/codes.html