server will not start (Debian)

I just went to do soem work on our test/sandbox instance. I had upgraded
this ssytem to V12 a few days agao. I found that it was not running:

Here is the last thing in the log:

2019-12-06 14:16:06.149 EST [3764] LOG: received fast shutdown request
2019-12-06 14:16:06.154 EST [3764] LOG: aborting any active transactions
2019-12-06 14:16:06.160 EST [3764] LOG: background worker "logical
replication launcher" (PID 3771) exited with exit code 1
2019-12-06 14:16:06.162 EST [3766] LOG: shutting down
2019-12-06 14:16:06.204 EST [3764] LOG: database system is shut down

This was in the logfile that was one rotation back. The current one is
zero bytes:

pg_lsclsuters shows:

Ver Cluster Port Status Owner Data directory Log file
12 main 5432 down postgres /var/lib/postgresql/12/main
/var/log/postgresql/postgresql-12-main.log

systemctl status postgresql shows:

root(at)stantest:/var/log/postgresql# systemctl status postgresql
postgresql.service - PostgreSQL RDBMS
Loaded: loaded (/lib/systemd/system/postgresql.service; enabled; vendor
prese
Active: active (exited) since Fri 2019-12-06 14:22:13 EST; 3 days ago
Process: 371 ExecStart=/bin/true (code=exited, status=0/SUCCESS)
Process: 5555 ExecReload=/bin/true (code=exited,
status=0/SUCCESS)
Main PID: 371 (code=exited, status=0/SUCCESS)

Dec 10 06:41:19 stantest systemd[1]: Reloading PostgreSQL RDBMS.
Dec 10 06:41:19 stantest systemd[1]: Reloaded PostgreSQL RDBMS.
Warning: Journal has been rotated since unit was started. Log output is incomple

What do I need to do to get it started AND fix whatever caused this
unexpected shutdown?

--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin