It would really depend on the services that you are using on your server. Let’s say if you are running Nginx, you would need to make sure that Nginx is up and running:
systemctl status nginx
If it is not running you would have to start it:
sytemctl start nginx
Then also make sure that it is enabled so that in case your droplet gets rebooted again it would start automatically:
systemctl enable nginx
If you are running your site in a container, make sure that the container is up and running:
docker ps -a
If the container is not running make sure to start it and also check your logs for any errors.
If you provide me with more information about your setup I could try and give you more specific suggestions.