Hey 
Check out your config/databases.js file in your strapi backend project. If you not changed something yet you will find that your data is actually stored on you computer in your strapi project (.tmp/data.db). For production deployment, you shouldn’t just copy/clone the file and put it on your server…
You should configure your config/databases.js according to the database you are either running yourself on your Ubunto or cosnider using a DB hosting provider and configure it (something like https://www.elephantsql.com/)
Also, check out the strapi on Heroku (with PostgresQL) tutorial here: Heroku Deployment - Strapi Developer Documentation