Car has no power at all

The first thing would be to have the battery load tested as it sounds like it has either discharged or failed. Once you have a known good battery you will have a starting point as it could be something in the vehicle draining it or it could be just a failed battery. However to test you need a good battery first. Many parts stores will test a battery for free if you take it to them.
If you look at the battery terminals they could also be the issue if they are covered with corrosion or damaged. This guide explains how to clean them.
https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked