One WP in domain and one WP in subdomain

I installed a WP blog under the main domain in the public html folder. This was a success and everything worked as expected.

Then set up a subdomain and installed another copy of WP for a related blog. That worked as expected without problems.

However, now when I return to the initial blog using the main domain name I get an error message saying
“Error establishing a database connection”

Trying to get into the dashboard also produces an error message
“Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at localhost. This could mean your host’s database server is down.
…………..
“

My question is:

Is it permissible to have one installation of WP in the public folder and one in the subdomain folder.

If it is are there any other likely causes of this problem.

The server appears to be okay, the subdomain blog is presenting correctly.

Thanks