if the battery tests OK, you might be sure your battery posts connections are clean and tight. as it wont start if its not getting good connectivity. next have alternator checked, that is free at any autoparts store.if its still not starting it could be fuel filter.the key is to rule out simple things first.after that you could be looking at electronic issues. most likely is loose connections or corroded battery cables which can easy be fixed with wire brush and a light coating of vaseline. Sometimes the starter can go bad and cause issues
the alternator has a built in component called a voltage regulator and that can cause the alternator to quit. If you replace things from autozone or checker they have lifetime warranty as part of the price- so if it happens again your covered good luck!
Depending on where you live,salt has a nasty way of eating through the wire connector to the starter,also if it's been cold the battery connections may have contracted and expanded enough to give you a poor connection try tightening them.
You have not given enough info. When you tried to start it what did it do? click. no click, crank, crank sputter, grind? Is the CEL on? Has it been running OK lately. Have you had any repairs done lately? How many miles on it?