You need to take a complete SQL backup of the database and a complete copy (usually as a zip) of the all the files beneath the web root. That will provide everything you (or any developer) needs to later make new copies of the site offline or on another host.

Most hosting account control panels have utilities for doing this.

Its best to disable and clear the c5 cache and disable pretty URLs before making the backups.

If you are on a 5.6 site, you can also use my Backup Voodoo addon to do this, but if you are shutting down a site and are making a one-off copy for your archives, you probably just want to use the control panel rather than purchase and install a new addon.

You may also want to make arrangements to keep ownership of any domain names. If you just close the hosting account, you could loose any domain names associated with it, so will either need to reduce it to a domain-registration only account or move your domain names to another provider's domain only account.

Good info at
https://documentation.concrete5.org/developers/installation/moving-a...

A gotcha on Windows installations is that the database may default to being case-insensitive. That can create problems when moving back to linux.