Yesterday I stopped by my local home depot while driving the XK, the top was down and I was enjoying the lovely day! After a little shopping, I tried to start the car, pushed the start button and nothing happened. Tried several times the same result (top down, new battery, foot on the brake peddle, etc.). I had limited trouble-shooting capabilities at the time. Removed the rear fuse panel cover, could hear a relay energize when I pushed the start button. Swapped this relay with another (of the exact type located in the same fuse panel), had no effect. Checked the fuse for staring located in the engine compartment fuse panel, it was good. Was about to call for wrecker transport, when I decided to raise the top and secure the car. Once the top was up and all windows closed, I tried pushing the start button and to my surprise the engine started normally? Obviously, I drove straight home and parked it in the garage. Now I'm making this post. Anyone experience anything like this? If so please share your experience.

 

I have a '09 XK with a good battery and hooked up to a CTEK 3300 when not in use.
I was out and about running errands on a warm, sunny day. I parked at a grocery store, locked her up, went inside...and turn right around when I realized I left my wallet in the car. I had my key fob in my pocket and grabbed the door handle as usual. Car door unlocked and opened, but I noticed the window did not drop. I gently closed the door and attempted to start the car. Nothing. No start, instrument lights, or Jag logo on infotainment screen.
Climbed out of the car, closed the door and waited 5 minutes. Climbed back in and she started up like normal. Only happened just that one time (knock on wood). I figured that the car was still shutting down and a glitch prevented me from starting the car. Have not been able to replicate the problem.

Perhaps she was just telling me she was tired and wanted to go home.

 

Same happened to me one day, stopped at a shop, when i came out no start, everything was dead. Thought it was the fob, so i was half a mile from home and a friend was leaving me home to get the spare fob. As we pulled away the car came back to life, alarm blazing. Pressed the off button on the fob. I got back in and off it went. These cars have notorious electric issues, so never panic. Just be patient with your cat. I always keep it plugged into my cteck charger in went not in use, so if you haven't got one buy one . The battery needs to be regenerated for optimum operation.

 

A lot of the modules don't power down & reset until you lock the car. So, if one of them doesn't boot up correctly, the only way to reboot it is to lock & unlock - in some cases (because not all of them power off immediately), lock, wait a few minutes, then unlock.