Lost all power need help

So I just bought a 1970 Duster 340. I took it out today for the first time since I purchased it and I lost all power. I got on it a little bit then everything shut off. I have no lights no nothing. What do you guys think the problem is? I think it might be the fusible link? This is my first classic car so I'm not really sure. Any help would be great. Thanks.

It probably happened too fast, but on a stock system if the ammeter swung over to full discharge that's a warning that fusible link will likely fail in a short time.

No lights pretty much means no power from the battery.
Engine off - Key off or key on, when the headlight switch is on the ammeter should show battery discharge.
Here's summary about Understanding Charging Systems with Ammeter

Anyway. Start with the battery.
Check the cable connections - both ground and positive.
Then the most basic check with a multimeter is voltage (with no load) from positive to negative.
If it 's under 12.5 Volts, see if it can be charged up.

Take the multimeter and leave the negative clipped to a ground,
measure voltage at the starter relay, then the back of the back of the bulkhead connector where the feed wire (witrh the fusible link) goes.
You can also check the BAT terminal on the back of the alternator. They all should be at battery voltage if there are no breaks.
If so, turn on the headlights and check again. Once there is current flowing, then weak connections will show up as voltage drops.
However I think you'll find the problem before having to do that.

The diagrams on the page I just linked should make it clearer what I'm referring to.
The next page has an overview of fusible links
Fusible Links in Charging Systems with Ammeter