1st things 1st.
Could be battery dying,
could be starter dying which required more voltage than even a good battery can put out to start the car,
could be the alternator dying not charging the battery.
Could be a parasitic drain of the battery.
Probably only one of the above.
Check your battery volts. Should be something like 12.6 when not running and 14.4 when running.
[This message has been edited by 2.5 (edited 11-14-2013).]