Originally Posted by
Thymeclock
All the advice given is accurate. Here's what I would do if I were you: put a battery charger on the battery overnight to bring it up to a full charge. (If you don't have a battery charger, buy one - it's worth having.)
After the battery is charged, if the dome light is bright and the engine cranks rapidly, the car should start easily. If it doesn't, it's a defective battery. Once the car starts, drive it around for a while, preferably with the headlights on. If after an hour or two of running the radio starts going out and things like the lights start growing dim, it's a bad alternator.
If nothing bad has occurred after those tests and the battery goes dead or becomes weak overnight, it's because something is drawing power (like a door left ajar or some accessory that is drawing current while the car is not running).