What causes car battery to hold only 5v?

One of my cars was parked for a few months and it couldn’t start after that. I could jump start the car and gave it some good drives (2hrs in total) to recharge the battery. However, that didn’t help as once the car engine is shut down the car won’t start as normal.

Checked the battery it is supposed to hold 12v but now only holds 5.2v max right after the drive. Alternator is good giving 14v when engine is on. Tried some car charger to charge the battery directly but it only recognise the battery as a 6V battery (as oppose to 12v).

Two questions: 1. is there any other way to restore the battery life? 2. what caused the battery to hold only 5v (and being recognised as a 6v battery) after parked for a few months? (I am interested in the internal mechanism)

Thanks mate