I have a 2003 Ford Thunderbird. It has been sitting all winter in the garage and I forgot to start it. Now, when I start it, it starts quickly then shuts down quickly and the horn blows.

The anti theft system maybe? If you have another key try that. There's a chip in the key that communicates with the onboard computer and other than the horn blowing, it sounds like that. One other trick is to disconnect the battery for about half an hour. That clears the ECU (computer) That last on the list.

I have the same problem my mechanic touched the neg cable to pos cable on the battery to shut the computer down and reboot its self still doesn't always start Seems if I put the key in and then push the unlock button on the key pad that will work three out of five times

I have begun experiencing the same issue, tried the other key, car started normally . I also charged the battery overnight this may also corrected the condition. Hope this helps.

After using the other key a couple of times, the issue of car alarm going off and engine stalling returned. I then removed battery,charged it fully and took it in to be load tested. Was told it was only putting out 60cca instead of 850cca. Replaced battery no repeat of problem.