Botched migration from 9.4 to 9.5 and I'm not sure how to fix it
Since you had 2 instances installed, the first (I guess 9.4) was using the default 5432 port so the 2nd got another port (probably 5433). When you uninstalled the first instance, the 2nd doesn't know anything about it and still uses the 5433 port.

You can use pg_lsclusters to find what instances have been installed, and some more details like whether the are running or not, their ports, their data directories, etc. Example:

ian@jupiter:~$ pg_lsclusters
Ver Cluster Port Status Owner    Data directory                 Log file
9.3 main    5437 online postgres /var/lib/postgresql/9.3/main   /var/log/postgresql/postgresql-9.3-main.log
9.3 test93  5438 down   postgres /var/lib/postgresql/9.3/test93 /var/log/postgresql/postgresql-9.3-test93.log
9.4 main    5435 online postgres /var/lib/postgresql/9.4/main   /var/log/postgresql/postgresql-9.4-main.log
9.5 main    5432 online postgres /var/lib/postgresql/9.5/main   /var/log/postgresql/postgresql-9.5-main.log
9.5 repl    5439 online postgres /var/lib/postgresql/9.5/repl   /var/log/postgresql/postgresql-9.5-repl.log
9.5 stage   5433 online postgres /var/lib/postgresql/9.5/stage  /var/log/postgresql/postgresql-9.5-stage.log
9.6 main    5434 online postgres /var/lib/postgresql/9.6/main   pg_log/postgresql-%Y-%m-%d_%H%M.log
9.6 repl    5440 online postgres /var/lib/postgresql/9.6/repl   /var/log/postgresql/postgresql-9.6-repl.log
10  main    5441 online postgres /var/lib/postgresql/10/main    /var/log/postgresql/postgresql-10-main.log
ian@jupiter:~$ 


So the solution is pretty simple. Tell pgAdmin to connect to port 5433 or whichever port your 9.5 has been setup to listen to. 

Alternatively, change the configuration of your 9.5 (second and now only one) instance to use port 5432. You need to edit the postgresql.conf file and restart it.