Getting Issue "Error establishing a database connection" On DigitalOcean

I have some issue like " Error establishing a database connection" on my website please look at the issue and give me the solution

i have tried with ee site restart reviewsilk.com --db
but its not working.

i have tried also sudo service mysql restart

it shows mysql not found.

please check the issue.

site: https://reviewsilk.com

Updating your server fixes this issue(almost all the time for me). Login to your VPS via ssh and run following commands to update and reboot.

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo reboot

I have done this now site is working perfectly i mean homepage is working perfectly but inner pages have same issue like “error establishing database connection”

I always solve this issue with: ee service restart nginx-proxy && ee service restart db