sounds like the battery is dead. could be other problems (corroded cable ends, you left a dome light on, headlites left on, for a few examples).

As to why the battery is dead...try these tests...

charge batt and make sure car starts and runs ok.
disconnect negative battery cable and let teh car sit for 3 days. reconnect battery and start car. if all is ok, then the batt is most likey OK (or you can have advance auto parts test it for free). It may not be holding a charge.

opsion 2, and some folks may yell at me for this...cahrge batt, start car, and while car is running, disconnect neg cable. if car stalls, then teh alternator is bad (or regulator, in the alternator so you change it all at one time).

If you have a volt meter, you can do a bit better...
a charged resting battery should show about 12.7 volts.
start eh car and check it again - it should be 13+ and could be as high as 14.5 volts. Try revving the engine a bit, and you should see the voltage rise (1500 RPM is plenty).

YOu can turn on everything electrical (wipers, defroster blower, hi beams) and the voltage shoud stay at about 12.5 or higher with the car running.

If all that check OK, then you prpbably have a drain on teh battery. There will almost always be some drain (radio memory for one thing). Simplest test is to connect a test light (12v bulb and 2 wires) between nag battery post and neg batt cable (cable is off the battery for this test). If the light lights, then something is drawing power. Now, remove the fuses one by one and check the light after every fuse. IF the light goes out youhave foud the circuit that is drawing power. if more than one circuit, then one will dim the light and one will put it out.