2005 V8 alarming and not turning over

Hi guys

I’m here out of desperation! i have a 20yr old S-type, low mileage, lovely car but it won’t start. I don’t use it that often and the battery has a tendancy to run down over a period of a few weeks. Last week, i had to jump start it - it started fine - so i connected a smart charger. When i went to disconnect the charger, i accidentally pulled the lead off the battery and when i reconnected, the alarm went off constantly. To avoid annoying the neighbours, I’ve disconnected the horn and found that the alarm stops after a while, but the car still won’t start - when i turn the ignition key, the lights just go out. And nothing works except for the radio and, sometimes, the heater. When i click on ‘climate’ the infotainment screen is saying “ignition not on”. Any ideas?

Hi Timothy,
Sorry no-one has responded to your pleas for help; there is unlimited knowledge here about the older models, less so the later ones.
I don’t have much either, but hopefully we can get the ball rolling.
Did you accidentally remove the battery lead while the engine was running?
\Have you fully charged the battery and just connected the two battery leads together for 30 secs before reconnecting to the battery?
Do you have access to an OBD2 scanner?
It is possible that when\if the battery was disconnected whilst running, the alternator produced an overvoltage and fried some electronics. Hopefully not enough that the scanner won’t point you in the right direction.
Maybe someone with more knowledge will chime in.