Short answer
Open your server in the VPS Panel and use the Console. It works even when SSH and the network do not, so you can check the firewall, the SSH service and the network settings.
Checks
- In the VPS Panel, the server is running. Start it if it is stopped.
- Sign in through the Console. Reset the root password from the server's options if you need to.
- Check the firewall still allows SSH (for example
ufw status) and that the SSH service is running. - Check the network settings were not changed. If the server is beyond repair, a reinstall gives a clean system; back up first.