Citroen Xsara Picasso - Engine stopped turning over - Spaniard

Hi hope you can help. I've got a Picasso 1.6 LX 2003(52) and all of a sudden the engine won't turn over. It was running fine then wouldn't turn over. The date/time has reset to Jan 2001. Also there is a beeping sound when I turn the key with messages about airbag fault, immobiliser and low fuel (although it has half a tank). I tried to jump it but no luck. I've heard various opinions that it could be a completely dead battery, the immobiliser or the starter motor. I don't know if its relevant but there was a very heavy downpour at the time. Thanks in advance.

Citroen Xsara Picasso - Engine stopped turning over - elekie&a/c doctor

Battery or connections sound favourite for this problem.Does this car have battery under passenger seat?