1972 Cadillac Deville will not start

You have a bad battery cable connection. To find where it is, get it to go dead again, then the goal is to keep it that way so there's a defect to be found. Don't try to get it working again. Turn on the head lights so some current is trying to flow. Now use a test light or voltmeter and start right at the battery posts. You should find 12.6 volts. Now move the meter or test light probes to the cable clamps, one at a time. You're looking for the first place the voltage drops or the test light is dimmer or out. If you have GM's miserable side-post battery, those prevent you from testing at the cable connections so you have to continue the tests further down the line. Most of the time though the bad connections are right there at the battery. Check for corrosion on the contacts and be sure the bolts are tight. Also be aware that if there is corrosion on the contacts that will prevent the battery from charging properly from a battery charger, so the battery may not be fully charged.