You need to restore the backup copy of your Metabase application database as well as rolling back the application code.
Background: On each upgrade you see (in the log at first startup) that Metabase typically performs some automated “migrations” with needed database schema changes so it can handle new features. An example is that collections in v0.30 now can contain dashboards and pulses as well. Sometimes these schema changes are just additions of fields, sometimes data is also moved around
If you lost your backup copy of metabase.jar at least in the summer of 2018 you could “get back in time” like described here: Metabase Updates ( SOLVED )