postgis installation gone awry [cross-posted from stack ove
On a Ubuntu 14.04 staging server a MySQL connection protocol had to be installed for full-text searching for a separate application.  This led to installing, uninstalling ad installing anew mysql and sphinxsearch.

After a reboot, another application hits an error because:

ERROR:  could not access file "$libdir/postgis-2.1": No such file or directory


in psql (version 9.3.10) the same error message is getting confirmed upon 

SELECT PostGIS_full_version();


notwithstanding that a check on the database returns extension "postgis" already exists.. whereis postgis also returns a blank.

Has postgis been somehow uninstalled? 

Attempting to install anew:

sudo apt-get install postgresql-9.3-postgis


getting Package postgresql-9.3-postgis is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source E: Package 'postgresql-9.3-postgis' has no installation candidate

Even attempting to create /etc/apt/sources.list.d/pgdg.listwith 

deb http://apt.postgresql.org/pub/repos/apt/ trusty-pgdg main  


then

wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
sudo apt-get update
sudo apt-get install postgresql-9.3-postgis


getting the same no installation candidate. Is this server hosed for postgis now?  Would re-installing postgresql impact existing applications on the VPS?
Try

sudo apt-get install postgresql-9.3-postgis-2.1


http://packages.ubuntu.com/trusty/postgresql-9.3-postgis-2.1