For God sake, how do I enable auto-save in ONLY OFFICE COMMUNITY EDITION? I’ve just lost the entire content of an unsaved document after my PC restart by itself while I was writing.

Please provide more details about the product and the versions of components you are using.
Also specify the OS of the server and the type of installation of ONLYOFFICE product (docker, deb/rpm, exe).
Note that there is a special script designed to safely shut down the Document Server to prevent data losses ( Sudden server restarts without running the script will result in data losses.