This error will usually be down to the site not being able to access the data in the database the site points towards. The most common cause is that the wp-config.php file has an incorrect table prefix.
To check this go to the public_html folder for your hosting and open the wp-config.php file, on line 62 (or thereabouts) there is a variable that sets the table prefix. If this does not match the prefix on the tables in your database then although the connection to the DB has been made the data cannot be read meaning the script will revert to the install option as though nothing had been installed there:
This setting needs to match the table prefix in your database, this is case sensitive so even something as simple as "wp" vs "Wp" would cause an issue :
If this does not match then go back to the wp-config.php file and update the value in the $table_prefix variable to match that used in your database.
