It could be a few things... the first top three are:
1-battery is crap... doesn't hold a proper charge
2-alternator is crap... not charging the battery properly and not providing enough power to run the car once the battery is too dead
3- the car starter is crap, without this, the car will not start.
The easiest way to check these... is get a boost, a charged battery or a portable power pack (with boosting capacity)
If the car does start, then it's 1 or 2.
If the car starts, then dies the moment (or within moments) of removing the extra power (other battery/boosting cables, etc.) then it's probably the alternator.
If it starts and runs, but won't re-start after you stop the engine, then it's the battery.
Now, these are the usual top three guilty factors, but it could be wiring that went bad or other things.