Had to have a new rear windscreen installed by.

Had to have a new rear windscreen installed by autoglass. Car was working fine before this but now won't start at all. Its reving over but won't fire up. Took the battery off which was completely flat after only 2 days of no use. Tried jumping it and charging it overnight but it still won't start. Do you think its the battery that needs replacing (its 3 years old) or could having the window replaced incorrectly have caused a fault causing the the engine to not start. Its was running really well before the windscreen incident. Any advice would be appreciated. Many thanks