My Mustang won't turn over?

It sounds like your battery is very low. Charge the battery and have it tested along with the alternator to make sure it is charging. Batteries don't last forever and do need to be replaced every few years.

Bad battery, good but discharged battery, or loose or dirty battery cables.

Was that real fast repeated clicking or just one single click each time you turned the ignition switch?

Grab an inexpensive digital voltmeter and measure the battery voltage. It should be 12.6 volts if it's fully charged.

If you find closer to 12.0 volts, the battery is good but discharged. Either something drained the battery while the engine was off or there's a problem with the charging system. The next step is to check the charging system. The battery voltage should rise to 13.75 to 14.75 volts with the engine running. If it's low, we have to diagnose that system.

If the engine will not start with jumper cables hooked to another car or with a battery charger, and it makes the same noise, that leaves loose or corroded cable connections.