Sounds like the alternator is no longer charging the battery.
I'd check that the auxiliary belt hasn't broken. If it has renew the belt, recharge the battery and you should be ok.
If the belt is still intact then it sounds like alternator failure but they normally last for 100k miles and more. What mileage has the car done? If the alternator has failed then replace it.
If the belt and alternator are both ok, check whether any earth leads have broken. If one has then replace it.
Failing all of the above then the battery could have failed. They normally last 6 or 7 years and if it is older than that it may have had its day. Check the battery connections first though. They could have come loose.