If it doesn't even crank then as Naki said, it might be security modules. Alternatively, right in the passenger footwell, up behind the carpet on the left side you will see a huge plug going into passenger inner wing, if this is loose and if it cut out you will get exactly the same thing as you described. All the lights on on the dash but nothing else!
1997, 4.0 LWB X300 - Unlucky car but still a daily driver.
1988, 3.6S XJS - Never ending restoration project.
1994, 3.2S X300 - Accident damaged, October 2015. Gone!
1994, 3.2S XJ40 - Long gone.