Before doing these, realize and know they are -extreme- actions which you normally should not do... and if you don't know how, you need someone who does to help you.
First, back up the whole data directory (you will have to find out where and how).
Next, purge the postgresql package (which removes config files and possibly database)
Then, install postgresql again
Finally, stop postgresql, remove the existing data directory, restore the backup of the data directlry and restart postgresql
That -might- solve your problem.