Sounds like either a fuel or spark issue. What year is your XJ?
For a quick fuel test, the first time you start it, prime the pump by turn the ignition but not all the way, do this about 3 times, then try the ignition. If it starts, then either a bad fuel pump, or maybe a bad fuel line.
For spark, do you have any other performance issues. Like cutting out, stalling, not starting after driving it for about 5-10 min.