check the starter, or maybe check the battery may be enough voltage for lights, but not to crank the engine over.
A bad battery or faulty battery connection.
What are the symptoms? Will it turn over? Can you turn the headlights on and are they bright? IS there a clicking sound? Did you clean the battery connections? The side terminals really need to be clean to send the best current to the starter especially in the winter. Try cleanign the battery connections FIRST before you go any further then check back with another posting on what you have checked.
A weak alternator and/or battery.
I'm not a mechanic / technician but if your battery is overcharging or the headlights are too bright so people are flashing their headlights at you when you are just using low beams ( it sounds like a voltage regulator problem )
Problem with the charging system. Altenator, Battery, etc.. (may even be a loose belt causing the alternator not to spin fast enough to charge the battery).
first check battery, then remove cables, polish battery terminals and cable connectors, do same at solenoid and all terminals with fine sand paper, steel wool etc. until bright & shiny.
turn the key? no start? check the battery connections. clean them make sure they are tight turn the key no start? turn on the headlights if you have bright headlights, the starter may be the issue. if you have no headlights, look at charging the battery after battery charge turn the key tries to start or clicks probably a dead battery if the car turns over and just won't start do you have gas? If the car turns over and tries to start that means the battery and starter are good now you need to find out why you are not either getting spark of fuel.
Have your Alternator tested. Sounds like the entire engine is running off just the battery.
Start by checking the battery voltage and cleaning the connections to the battery. You can use a voltmeter to check voltage or just turn on the headlights and see if they are dim or bright (low voltage = dim headlights). Once battery is checked and your sure it's not the problem, remove the starter and take it to an auto part store. Many auto part stores will test the starter for free. The clicking can be a lack of battery power, poor connection or faulty starter.
according to Florida handsbook, you must dim your bright headlights within 300 feet of the vehicle ahead.
check that the wire on the sylenoid is hooked up. also, check for clean connection to the battery terminals. I have one that looks clean but if it's not bright and shiny, it causes problems.
If it does nothing at all when you stick the key in..it could be the battery is totally dead. If there is a "clunk" sound when you turn the key..it has to do with the starter, solenoid, or related to the starting area. It could be the battery still (not charged enough to start the car). Turn on your headlights and see if they are dim, won't light, or are bright. If they are bright, it usually is not the battery. No gas. Dead battery. The possibilities are numerous. Start with the easiest and go from there. Is there fuel? Is there power from the battery to make starter turn and make the spark plugs spark?