The database hold details of your Projects. Here is a quote from the manual:
"Unlike other applications which save self-contained project files to user-specified locations
wherever you like in your file system, DaVinci Resolve takes a more centrally organized
approach to project management, using databases. By default, DaVinci Resolve uses a disk
database to keep track of every project you create. The Database sidebar lets you manage the
projects found within this database, which are saved to a specific directory on your system
(particular to that database). The default location of this disk database depends on the
operating system you use."
The Project database is the first page that opens when you start Resolve and it lists your currently stored Projects.
Database.jpg
Now, if you didn't make a copy of your version 16 database and updated it to 17, it will not open in version 16 as it is not backwards compatible.
From the blue colored Read more tab on the downloads:
"Important information regarding database management
DaVinci Resolve 17.0 requires a database upgrade from DaVinci Resolve 16.2.7 and previous versions. We strongly recommend that you back up your existing database (both DiskDB and PostgreSQL) before performing an upgrade."