1994 Vauxhall Astra 4 cyl Two Wheel Drive Manual

we have had this car for only a week now and at first it took a while to start of a morning especially if it was cold and sometimes while trying to get it started the battery would go dead. but some lads tried to break into it the other night so we unconnected the battery in case they came back but then in the morning when we re-connected the battery the car was dead, the lights work ok so far but when turning the key you feel nothing at all. what could this be we wounder if we need a new battery or not. thanks????