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.
