To install the latest version of Strapi use:
npx create-strapi-app@latest my-project --quickstart
Until the migration guide for Strapi v3 to v4 has been released you should not try to upgrade your current Strapi v3 project just yet. The migration guide will roughly be released at the end of Q1 2022.
As for your current situation, if you have a database backup I would try to put that back and also revert your code to before you tried to update.
Hope you get you project going again!