You may have too much corrosion on the terminals or cables, or it could be a problem with your starter/solenoid.. there are a plethora of problems that would cause exactly what your talking about.. Any other details?

 

you probobly need a fuel pump, I just put a pump in an XJ8 this afternoon for the same problem, crank it and check for fuel pressure on the fuel rail by depressing the shrader valve with a small screw driver, no pressure. replace the pump. You access it by pulling up the back seat, pop out the black rubber/plastic round panel under the passenger side rear seat. And you see where you need to remove the fuel tank access cover.