Car won't start

Try turning the crankshaft by hand to see if the engine is locked up.

If you heard and felt it clank to a stop, you have a mechanical problem, most commonly a jumped timing belt, and valves are hitting a piston. Normally the starter motor is strong enough to overcome that and bend the valves, then the engine will crank too easily from loss of compression.

If the crankshaft becomes gradually harder to turn, with no sudden stopping, you're likely coming up on top dead center with one of the pistons. The compression will bleed off fairly quickly, then you'll be able to turn it some more.

Back the crankshaft that quarter turn, then try cranking it like normal. If something is locking up, the engine should crank that quarter turn, then stop. If it still doesn't crank at all, use a digital voltmeter to measure the battery's voltage, first while doing nothing, then while a helper momentarily tries to crank the engine. Have them do that just long enough to get the reading. Tell me what you find.