Pull the radio fuse and leave it out for a few minutes. While it is out check it even though it may look good. If you have a meter or a 12v tester lamp check the hot side of the fuse socket. Check all connections to the radio.

 

By all means, check the fuse. 2001 XJ8 Radio receives power from fuse #9 in the left hand heelboard fuse box, according to:

http://www.captainjaguarscathouse.co...ec%20Guide.pdf


Did the display show 'CODE REQUIRED' or something similar, when pressing any button? - when battery has been disconnected for a time, some Jag radios require a security code to be entered to operate again. Not all US models require this, but my 2000 XJ8 does. The dealer can give you the code via phone if you give them the last digits of your VIN.

 

Almost certainly you'll have a keycode radio. As Quadmaniac says if the battery has been disconnected (even for a second) to safely do a job on the car, the radio will lose its code. The LCD display will say "code required" or somesuch. If you look in the black wallet that comes with the car (or should) the radio book should have the code on a plastic card, like a credit card. But these get lost, so call the dealer as he says. If the battery has been disconnected for some hours, all the station settings will have gone too.
Keycode radios became the norm after so much thievery of car radios. But since they became built in as part of the fascia panel, the display and computer complexities, not fitting anywhere else, it seems a bit pointless now.
Leedsman.