You’re talking about an e-liquid tank full of distillate, kind of like this, right?
If you just filled it, you should just have to let it sit for a while - I left mine overnight before I hit it to let the distillate soak into the coils.
If it worked for a while before dying, though, then the atomizer might have burned out. You can replace it, but you’ll have to empty and refill the tank, so it might be easiest to just empty it into a spare tank and use that one for a while.
They also usually use some weasel words like “up to.” That way, if it doesn’t last the full 72 hours (which it won’t), they can claim that they stated “72 hours MAXIMUM” rather than just “72 hours.” It’s basically shifts the statement from “lasts three days” to “definitely won’t last four days.”