Ok....2003 XK8 was garaged during BRUTAL Jan/Feb months in Detroit. Want to move it different storage location, but it wouldn't turn over. Roadside assistance came out, but charging unit would not let car turn over. Lights were on; it just wouldn't start. Bought own charging unit; plugged it in and left it overnight. Next day, NO lights will come on even though charger says battery is 100% charged. COMPLETELY dead! Having it towed to shop, but what do you folks think? Could this be a security/alarm system issue?

 

Much more likely a connection in the electrical distribution system. I would start at the battery and check/tighten connections, both B+ and ground, outward.

 

Try to disconnect for 60 seconds and then reconnect the battery before you do anything else...it might just bring it back.

Those portable start units should be banned from use with Jaguars...they do not provide sufficient voltage under load and can 'crash' the car's computers. Disconnecting and reconnecting the battery will reset them.