remote connection refused

I'd like to have a checklist upon what to do, what to investigate, when an
external connection is refused.

if i connect by :

psql -h localhost -p 5432 -U yt mydb

it works as usual

however even locally but using the LAN address :

psql -h 192.168.0.20 -p 5432 -U yt mydb

i got a :
psql: could not connect to server: Connexion refusée
Is the server running on host "192.168.0.20" and accepting
TCP/IP connections on port 5432?

obviously I've verified by ping the computer is reachable.

something i suspect :
$ lal /var/run/postgresql
total 12
drwxrwsr-x 2 postgres postgres 140 nov. 19 09:53 .
drwxr-xr-x 26 root root 900 nov. 19 09:45 ..
-rw------- 1 postgres postgres 6 nov. 19 09:53 9.1-main.pid
-rw-r--r-- 1 postgres postgres 4 nov. 19 07:10 pgbouncer.pid
srwxrwxrwx 1 postgres postgres 0 nov. 19 09:53 .s.PGSQL.5432
-rw------- 1 postgres postgres 71 nov. 19 09:53 .s.PGSQL.5432.lock
srwxrwxrwx 1 postgres postgres 0 nov. 19 07:10 .s.PGSQL.6432

this the locked file :
.s.PGSQL.5432.lock

is that normal or not ?

something to say, i had a brutal shutdown due to over heating of my laptop
recently.

on the pg_hba.conf i do have a bunch of IPV6 addresses allowing connection
then i don't think the prob comes from here, but nevermind...

in such a situation what is your checklist in order to find what I've
missed ?
--
Yvon