Turn on the headlights, and see if they work. If the headlights don't come on; the battery is probably dead. This could have happened because the battery failed from age; or because a light or electrical accessory was left on overnight; or it could have come from a short that developed in the alternator diodes, and then drained the battery after the motor was shut off. Shorted alternator diodes can sometimes draw enough power to blow main fuses or fusible links. When that happens; the whole electrical system will shut down, even when the battery is strong.
You can do a crude test to see if there is still power left in the battery by very briefly touching the two ends of a single jumper cable or a length of heavy electrical wire to the two battery posts. There will be a big spark when the wire is shorted across a battery that has power in it. If left connected; the wire will quickly heat up and melt the insulation, so don't let it touch for more than a fraction of a second.
If the battery is dead; it may be able to be recharged, or it might have to be replaced. Only by trying to recharge it with a suitably sized battery charger (6 amps or greater) for 12 hours or longer and then retesting it can it be determined whether the battery is still good.
If you recharge or replace the battery; be careful when you reinstall it. It is normal to get a small spark when the last battery cable is connected; but if there are no doors left open or electrical accessories turned on, and there is a big spark which makes an audible sound when the last battery cable is touched to the battery post; then there is a short either in the alternator, or somewhere else in the car's electrical system. In that event, do not leave the battery connected until the problem is repaired.